avatar: refactor avatar loading and caching

Merged Julian Sparber requested to merge jsparber/fractal:refactor-avatar into master
  • use the same method everywhere to display an avatar
  • move code to generate letter avatars to a separate crate
  • use the generated avatar as a fallback till the real avatar is avaibile

Note: The avatar in room details dialog is borken but the dialog will be replaced soon. Also updating the username in the room history doesn't work, the room history needs a refactor to resolve some issues.

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