Activities

Hook up the UI to the activity feed.

- [x] Comment activities
- [x] Share activities
- [x] Create content activities
- [x] Content visibility update activities
- [x] Group visibility update activities
- [x] Group update activities
- [x] Group creation activities
- [x] Adding a member to a group activities
- [x] Comment activities need tweaking after fixes in Hilary PR.
- [ ] Content previews need to be hooked up to the activity UI.
- [ ] Create a few more fall-backs for activities that don't have a preview image
- [x] Infinite scrolling of activities per 12