Change user avatar at PUT /api/v2/users/{user}/avatar

[raffis]

Describe the change

The user avatar should be replaced at PUT /api/v2/users/{user}/avatar.

Current situation

Currently the user avatar is changed by issuing a PATCH to /api/v2/users/{user} with a base64 encoded png image in a field avatar.

Should

• PUT /api/v2/users/{user}/avatar to change the user avatar
• Support for PATCH avatar at /api/v2/users/{user} should be dropped in the same release.
• The avatar should be validated as a valid image which is not yet the case.

Additional context

Also see #418.