Federate spoiler_text using warning attribute on <content /> instead of a <category term="spoiler" />
Clean up schema file from accidental development migrations

Do not autoplay videos, display play button instead. Use expiring links when using S3. Do not keep originals
for avatars/headers, resize avatars down to 120x120 instead of 300x300. Set cache headers on S3 stuff, also
make it private (aka only accessible via expiring links to prevent hotlinking)

Fix #233, fix #268

This might help with GS compatibility

as well as formatting of content

and atom feed generation to some extent, as well as the way mentions are
stored

relations on Status#mentions