StreamEntry model. Simply render Salmon slaps as they are needed

Filters out hidden stream entries from Atom feed
Blocks now generate hidden stream entries, can be used to federate blocks
Private statuses cannot be reblogged (generates generic 422 error for now)
POST /api/v1/statuses now takes visibility=(public|unlisted|private) param instead of unlisted boolean
Statuses JSON now contains visibility=(public|unlisted|private) field

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

(But there are no Atom feed pagination elements yet)