Closes: #345

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)

from accounts/tags because it's a terribly inefficient way to paginate
large sets of data

Fix #233, fix #268

by updating it at least every 24h if the user visits the site

This might help with GS compatibility

This prevents the bug where if you go "back" to the UI after navigating to
another page it loads with the old set of statuses

registration API

as well as formatting of content

Added stack trace for SQL queries in development
Removed badly thought out accounts/lookup API

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

relations on Status#mentions