Users module refactoring

Changes:
- functionality to edit users (currently: username + groups)
- allows any logged-in user to change the passwords of any other user
- improved url highlighting of subsubmenu
- added one test :)

Notes:
- This provides the basis for removing the expert mode as it uses django functions instead of custom 'add_user' etc.
I'd remove the 'expert mode' and the 'lib' module tomorrow and update this pull request. If you want to have removing the expert mode in a separate pull request please tell me as soon as possible.

- I made it possible for every user to change all passwords because I think of the freedombox as a "one person box". Which means I do not change the password of somebody else, but rather the password for myself, i.e. my SSH user account.