Use username over email address for authentication

Some users have expressed concerns over the required mail address as the main user identifier.

Although we don't do anything with a given email, having a username instead of the mail address seems to increase trust for first-time users.

Changing the user identifier from email to username implies:

• Changing the setup and login dialogue
• Extending the user model with a username concept while keeping the email field (that we can still use for sending notifications etc)
• Check the codebase for anything where we use the mail address for identifying users and change it to the new username concept