We can create a group
Add some more validation
Check that group was created
Create a members stream for groups
Some comments
Move feed validation function to activity test util
The group members feed is retrievable
Add an inbox link for groups
Fix activity-inbox link for groups
Add a group inbox feed endpoint

Add an inbox feed endpoint for the group inbox. This required tweaking
the activityFeed() function in lib/streams to take a context argument
rather than a user, which in turn caused a lot of downstream twiddling
with the user activity endpoints.

Also, create an inbox stream when the group is created.
Join a new group
Add an action for joining a group
Add a model for group membership

We keep a separate record outside the stream for group membership,
which makes testing for membership significantly easier.
Test adding a member to a group
User can leave a group
Leave a group
Change the lead comment
Two users join a group
Test for two users joining a group
One user posting to the group