Refine the account status button (with two states: Login/Logout).

[leewyatt]

Currently, in larger sizes, only "Login" is displayed, while in smaller / Medium sizes, only "Logout" is displayed.

[dlemmermann]

We have to really change this button. There should be no "logout" mode. We either see "Login" or we see the username and avatar of the user that is logged in. When the user clicks on the username or avatar then we get to see the user's profile information. In there the user will be able to log out. I am assigning this ticket to @FlorianKirmaier as he will be starting work on the login part soon.