view_river: simplify how entries are passed to the template
use unipath rather than pathlib

Server only has Python 2.6 which bails on some pathlib stuff (namely
using literal set notation).
install Fabric
settings.py: use sockets for production DB connection
install gunicorn
view_river.html: fix html_url link
fabfile.py: remove stray requires
settings.py: set STATIC_ROOT
Feed: update callback_url with new domain
add fixtures/feeds.json
Feed: use correct Superfeedr hub URL
feed/models.py: send auth when subscribing to superfeedr
Feed: automatically set current_state when using superfeedr

See comment for more information.
Feed: handle both HTTP 202 and 204 when changing state
handle_callback: quickly bail if no feed can be found