simplify (user)name in profile/ account

[scheidtdav]

Description

The application currently knows about a users email and an arbitrary name in the account.
In addition to that, the profile also holds a username.
This seems to be redundant to the point, that it may cause confusion as outlined in #645.

There should probably be only a single username either in the profile or in the user schema.

Use case

• Public self-chosen username for public profiles
• Displaying it in the profile/ settings

Implementation ideas (optional)

Additional information (optional)