We were already using signals for everything except delete, and even
in our old version of django the delete signal exists (it didn't exist
when this code was first written).
Django doesn't really like models to be OOP like this, so keeping PgModel
would cause issues with upcoming changes in django 1.8. Using simple functions
is easier, and the actual functionality is replicated straight off.
This is used so we can reference the group as a negative id number from
a template, which is the standard way we separate it from lists when
included in the same dropdowns.
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.
