[FIX-141/161] Budgeteer administration menu.

[maximAtanasov]

This PR should be ready for review now.

The administration menu is started as a separate web app (this one is set to run on port 8090) and
can be started from the root folder by running ./gradlew budgeteer-admin-interface:bootRun.
The first user to log in to it is made global admin.
The global admin currently has the following abilities:

• Set the password/email of any user in the Budgeteer
• Delete any user in the budgeteer
• Make any other user admin as well
• Delete any project
• Change the permissions of users in a project
• Remove/Add users from/to a project
• Edit the project name and start/end dates.

This adresses issue #141 and issue #161

[acetous]

• Benutzeradministration gebaut

[LeonieAdis]

Bugs
User administration

• E-Mail-Entry: the entry wants a string like "a@b.cd". If you don't enter the @-char and at least one char after it, a error message appears. If you enter "a@b" or "a@b.c" the input is not accepted and reset, but there is no error message or explanation, why this input isn't correct

Project administration in the administration application

• Clicking the breadcrumb "Project administration" leads to a NullPointerException.
• I would write "Project title", not "Project-title"

Project administration in the normal application

• I removed the admin status of the current logged in user. After accepting the warning, I was still able to access the project administration menu and change my status again. I set my status to an admin again and the button to the administration menu disappeared.
  After logging out and logging in again, I could reach the menu again and in the view of another admin, the first user was an admin, too.
• I would write "Project title", not "Project-title"

All pages

• the project title is called "Project-title" in the project administration, but "Project name" in the table on the bottom of the administration menu
• The input length of the entries isn't restricted, which leads to exceptions if you enter to long strings.

Usability

• I don't like the dropdowns for changing the role of a user in a project. One can either be a user or an admin and a user. I would suggest changing the dropdown to a checkbox, so one can either be set to an admin, as @tinne mentioned in Missing option to administer users #141 before. I even could break the application by unsetting both the admin and the user status.
• After changing the role of a user in a project, I would prefer seeing a message, that the new role has been saved successfully. There is a little bit too less feedback for the user. Initially, I don't know if I have to click a save button or if my changes have been applied automatically.

Styling/GUI:

• User administration menu: There are strange lines under the entry-button-groups.
• User administration menu: There are no labels for the entries and there is not much place for labels, so maybe placeholders in the entries might be helpful to know what to enter where
• User administration menu: the buttons don't have the same height as the entries, what looks unbalanced.
• Project administration menu: The "Save"- and the "Add user"- buttons have sharp corners and are completely flat, while all other buttons have rounded edges and a 3D effect on the bottom border.
• Project administration menu: The role-dropdown is higher than the button next to it

Additional comments
I tested this PR by running both the "normal" application and the admin application.
It wasn't possible to be logged in both applications at the same time. When I logged into the second application, I was redirected in the first application to the log in page after clicking something.
I tried using different accounts or the same account for the two applications.
I don't know, if that is intended or must be changed at all, I just wanted to document and discuss this "feature".

[acetous]

Hey @LeonieAdis, can you please adapt the suggested changes?

[LeonieAdis]

I implemented all my suggestions, except for the missing feedback messages for the e-mail-entries.
I know, a similar e-mail-entry with correct feedback messages is used in the "Register"-page, but I wasn't able to transfer that functionality.
I tried different ways, but couldn't figure out how to get it working.
Maybe someone else can help here.
In my opinion, the missing feedback messages aren't a big issue, so it may also just be kept like it is right now.

The earlier version of this PR was - due to addition of not-nullable attributes - not compatible with an already filled database. I fixed this by setting those attributes nullable.

Additional there was a logic mistake in the project administration:
If the current logged in user was a project admin, his admin rights could only be unset, if there was another project admin. But if the current logged in user wasn't a project admin and there was another project admin, the admin rights of this second admin could be removed, so there was no project admin left.
I changed this logic:
Now the application checks, if there is more than one project admin. Then all project admins can become non-admins.
If there is only one project admin left, you can't remove his admin rights.
In this way, there always has to be an project admin.
The only exception is a project, that was created in a earlier version of the application, when we didn't have user roles. Then by default, there is no project admin.
In this case, you first have to give admin rights to a user in the project.