Manage multiple accounts

[hecktimo]

storing multiple user accounts.
switching accounts and refreshing caches.

I would like to introduce a feature to manage multiple accounts in the same app.
This is a working example. It basicly replaces all references to a server object by either the UserController or PasswordsViewModel respectivly to access state.
Since the active account can be changed dynamicly now, which I added an AccountsActivity for, references to the active server as well as access to the accounts data must be managed.

Two more things I'd like to mention.

1. There is a test case failing: testSodiumSolve
2. If the account is changed and you return to the passwords screen, the avatar icon does not update. It updates if you switch screens between folders and passwords e.g.

If this is something you would consider worth looking into, let me know.

[hegocre]

Hello!

Thank you a lot for your contribution! I will take a look as soon as I have some time!

Cheers!