Overview
Let users choose a custom avatar to use for their profile in the fediverse.
Background
Users
Requested on this forum thread.
Implementation
Phase 1
When getting a user's avatar URL, first check the avatars directory for {collectionID}.png. If it exists, use that. Otherwise, there is no change in the logic.
Phase 2
Support uploading an avatar in the UI.
On the blog's customize page, if federation is enabled, show what it currently is (the first letter of the blog, by default). Include a file upload field next to it, and upload that file as {collectionID}.png in the static/img/avatars/ directory.