Add a "Join Group" button to the Organization Profile page

[joshuapowell]

Problem

We have three permutations of this functionality that should be available to the user depending on status and authentication:

• Existing User should be able to click a "Join this Group" button and instantly be added to the group.
• Existing User's who already belong to the group should receive the opportunity to "Leave Group" instead of being shown the "Join this Group" button.
• Anonymous or Unauthenticated Users should be able to click the "Join this Group" button and be asked if they want to Sign Up via a model window or given the opportunity to navigate away and Log In.

[joshuapowell]

The "Existing User" bullet points have been satisfied and are complete. See commit 5b3e41b for details.

Please refer to issue #215 for Anonymous or Unauthenticated "Organic" group joining.

Complete.