Add the active project to the URL

[zepich]

As reported by @MaPhiKo in #161, it's a problem if the user decides to switch the project while doing something, for example, opening a new tab while creating a new project.

We've decided to add the active project to the URLs to fix this issue. With this, the URL defines for which project an action is executed ('adding a new rule' for project 1, 'changing the project settings' for project 2).

The last active project (the project from the previous request) will be stored in the session to make it easier to return to the project if the user accesses the account settings or the administration.