VUMIGO-253 groups are timeout-prone, provide API

**Not ready for review yet**

The groups (specifically the smart groups) are very likely to create
timeouts in the webapp because:

1) they're unbounded by definition
2) we're trying to show the number of entries in a smart group in the Django UI
3) we're unable to page through the group members (same problem as the message display)

This PR is about creating an HTTP API in front of the group access where we can more efficiently query Riak & cache results (if necessary)