Commons_Groups main features

We should featurize the following configuration and ideally have it live as its own project on Drupal.org:

- Anyone can post content into any public group
- After posting content into a group, a user is listed as a contributor to that group (is assigned membership to the group)
- Likely a separate feature module: Users can Follow a group as defined by #1710688: Activity stream & Email notification system
- Single, comma-separated autocomplete text field that accepts autopopulation for specifying group audience
- Export a default group homepage that matches the Commons interactive prototype including:
-- the ability to hide the group description and have that setting persist on a per-group basis
-- the group content browsing widget defined in #1710782: Theme the Group Homepage browsing widget

Embedded information on Group node teaser pages:
- Recent contributors
- Recent additions

This needs discovery:
- "[x] contributors discussing the topics [topics]" text.

Specifically, how is that list of topics generated? Is it topics assigned to the node, or are we recording radioactivity per-topic within a group? Seems like we could ship with the former initially and then move on to the latter.

- Groups directory (https://projects.invisionapp.com/share/K43PUUHY?comment=true&conversatio...)

Note that related blocks around active content and groups are defined in #1710724: Default Radioactivity integration.