Group Enhancements 2 (Group Activity Feed, Fixes)

**Group Enhancements 2 (Group Activity Feed, Fixes)**

This PR adds a Group activity feed.  In many cases, a user would like to follow the output of a group instead of just a single user.  There are 2 use cases that immediately come to mind.  First, the primary point of contact for a theme or region is on leave so someone else on the same team fills in temporarily.  Second, a team rotates responsibility for producing a standardized daily dataset, such as the current demographics of a refugee camp.  The Group activity view checks permissions using the `can_view` permission and only includes public activities via `Action.objects.filter(public=True, actor_object_id__in=members, )[:15]`.  Additional fixes included in this PR are listed below.

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Fixes:
+ Adds help text for the group access field.
+ Layout fixes, including hyperlinking the group logo in the list view, boostrap forms, and other minor fixes.