Render a group's activity stream on its' page

[seanh]

Add logic functions for getting the activity stream for a group in JSON and rendered HTML formats, add HTML activity stream into group page.

[seanh]

This is finished, see the activity streams branch:

 https://github.com/okfn/ckan/tree/feature-1515-activity-streams

but I think that before closing this ticket a little more thought needs to go into what the activity streams and the group page should look like, where on the page they should go, what else should go on the page, etc.

Also maybe add a simple test that the activity stream is being rendered into the page.

[seanh]

Closing this as there doesn't seem to be interest in showing activity streams on group pages anytime soon