Add Follow/Unfollow group button
Add tests for Follow/Unfollow group button
Add links to sidebar navigation

I added links to the pages that exist so far. I omitted the link to the
privacy section of the user profile because we cannot yet jump directly
to it.
Fix privacy link assertion
Add icon-lock class when toggling privacy settings

The design uses icons applied with CSS classes rather than toggling the
visibility of a separate lock element.
Merge pull request #196 from azavea/feature/jcw/add-main-nav-links

Add links to sidebar navigation
Merge pull request #198 from azavea/feature/jcw/add-lock-icon-to-privacy

Add icon-lock class when toggling privacy settings
Merge pull request #184 from kdeloach/topic/follow-unfollow-groups

Add ability to follow/unfollow groups
Show group detail breadcrumb link on edit page
Move group detail URL construction to model

- This property is idiomatic to Django
- Removes the need to add an additional context item
- The Django admin will create hyperlinks if this is defined
Correctly set group editability context var