Add caching for avatars

As a User I'd like the avatars to be cached so that I don't download them every time

Acceptance Criteria

• Avatar is cached for some period of time (30 days)
  • Max Age is set to 30 days
  • Expires is set to +30 days from now
  • HTTP code 304 is returned for cached avatars
• Avatar is updated when user uploads new versions, but this may not be visible right away - some refreshes may be required.

Note, that browsers often cache images for 1 hour anyway, so you'll see 304 response in both cases.