Add support for usernames

[abainczyk]

As a first step in the direction of supporting collaboration between users in ALEX, we want to extend the user model by a username property.

Todos

• update the user model to have the field 'username'
  • the username should be unique
• create a component for the user registration form
  • it should be possible to specify a username in that form
• on the start page, one should be able to switch between login and registration form via tabs
  • if the registration was successfull, the view should switch to the login form and insert the email and password combination
• display the username in the top right corner in the header instead of the email prefix
• when an admin creates / edits a new user via the user management view, he should be able to specify / edit the username
• extend the table in the user management view to display the username of a user