Port to libQuotient's keychain and account handling

[TobiasFella]

• At the moment, the "Stay logged in" option is ignored and the connection is always stored in the keychain. I don't think libQuotient currently allows us to have this option
• When logging in, the "You're already logged in with this account; do you want to continue" [not the exact string] dialog was removed for mostly technical reasons; IMO this should be completely blocked before even logging in since there's no point in logging in multiple times with the same connection and libQuotient might explode from doing it :)

Also, I'm not familiar with the codebase, so this is done relatively blindely - please test well.

[KitsuneRal]

I forgot to address the "limitations":

• At the moment, the "Stay logged in" option is ignored and the connection is always stored in the keychain. I don't think libQuotient currently allows us to have this option

At first, I thought that this option should just go because with the current E2EE state (without SSSS and soft logouts) it's all too easy to lose the database. On the other hand, I personally am using it for temporary logins into test accounts... When E2EE is off, I guess it's as easy as making saveAccessTokenToKeychain() call optional. With E2EE though, I guess I would opt into an in-memory database instead of storing anything on the disc... Maybe something for 0.8.

* When logging in, the "You're already logged in with this account; do you want to continue" [not the exact string] dialog was removed for mostly technical reasons; IMO this should be completely blocked before even logging in since there's no point in logging in multiple times with the same connection and libQuotient might explode from doing it :)

Well, I tested libQuotient to work in this case, before E2EE :) and I see that E2EE doesn't really support it, binding the database to the user account, not to the device. I agree that it's a very exotic case and don't mind dropping it; but we have to add some safeguards in the login procedure then.