Bugs 938723 948491 951515 - Curated group viewing and joining

Implement joining groups that might require approval.
Add through model to group membership so we can track membership status.

Fix bug 948491 - join group
Fix bug 938723 - view group as curator
Fix bug 951515 - curated group IRC channel and member count not displayed
Merge pull request #748 from caktus/dan-wray-join-groups-combined-branch

[Fix Bug 938723] Dan wray join groups combined branch