Break up role concept

Leave the landing page as is (if you type in the key and enter, you are logged in as guest).

Leave the top right icon for the account, but just enable "log in" (without having to select a role).

On login, every user should land on the same view which shows:

• session enter component
• create room component
• created rooms
• moderated rooms
• visited rooms It should give the user a "home view" which allows access to the basic arsnova functionality (creating a room, reentering an already entered room e.g.) without having them to logout and re-login to change their role.

Note on mobile view:

A tab bar (or changing the current sub-component via button) could help managing all the different components on the home view. Joining a room should be prioritized on mobile.

---

Connected to #195 (closed)