Only allow logging in through the web frontend

[leschbenedikt]

Currently in the app a user can choose between "login" and "login with frontend".
We should consolidate this by only allowing to login with the web frontend.

After a succesful login the user will be redirected to the app via a custom vikunja:// app scheme

[kolaente]

The web frontend doesn't yet have the capabilities to authenticate other applications via oauth, so we'd need to build that as well.