Add user deletion ability for admins

[fpapadopou]

An administrator should be able to delete teams and/or users. They should also be able to
backup deleted teams' projects.

[fpapadopou]

The main goal is that an admin must be able to delete a team from the search results page.
Deleting a team should include the following:

• Create a zip with the team's workspaces and send it to the admin (optional)
• Delete the files of the team
• Delete the team from the database
• Delete the users from the database

The zip option will be added in a separate PR, encapsulating it in the deletion process is kind of dirty.

[fpapadopou]

Completed in #5, closing the issue.