Support for gravatar

[raffis]

Is your feature request related to a problem? Please describe

No.

Describe the solution you'd like

Add support to fetch the avatar from gravatar.
See: https://en.gravatar.com/site/implement/

Additional context

The avatar may be fetched from /api/v2/users/{user}/avatar which currently returns the internally stored gravatar. It should also be possible to fetch one from gravatar.