RethinkDB migration

[stuartpb]

Now that RethinkDB supports geospatial indices, we should look into swapping out MongoDB with RethinkDB (as soon as RethinkDB hosting is ready, which is to say Plushu addons are ready).

[stuartpb]

Plushu addons have been working for a while now, so this is just waiting on the app itself (and an export from prod).