Add middleware for redirecting from a microcosm subdomain to a custom domain

Where a site entry has a custom domain associated with it, all requests
to the microcosm subdomain for that site will be 301d to the custom domain.

This simplifies Persona audience handling for login (among other things).
Favour request.get_host() over using request.META dictionary
Add fabric task for restarting memcached
Activate redirect middleware
Simplify logging configuration to only use gunicorn logfile
Only log errors in production
Temporarily 301 to custom domain