Add geography management to app admin interface

[hancush]

Spinoff from #1025.

Sometimes, post geographies change (e.g., redistricting). We currently manage geographies behind the scenes, but it would be better for Metro to be able to manage this change themselves.

Add (or identify an existing) way for Metro to manage post geographies from the admin interface. N.b., geographies should have a start and end date, that way memberships to posts are associated with the correct version of the boundaries.

@fgregg I can't remember – did you start on something like this during the django-councilmatic 2.5 upgrade?

[fgregg]

I don't think i did, but we could!