Instead of requiring all users to log in to the main website once,
this allows them to keep using their old password on any aux sites
that uses the community login system.
We might want to consider actually migrating the accounts to the new
system at some point in the future, but for now, just allow logins
against the old tables, and have migration only happen when the
user logs into the main website.
This allows all models inherited from PgModel to specify which
URLs to purge by either setting a field or defining a function
called purge_urls, at which point they will be purged whenever
the save signal is fired.
Also implements a form under /admin/purge/ that allows for manual
purging. This should probably be extended in the future to show
the status of the pgq slaves, but that will come later.
Includes a SQL function that posts the expires to a pgq queue. For
a local deployment, this can be replaced with a simple void function
to turn off varnish purging.
