** WIP ** - Group join/leave

** This is a work in progress - not ready for merge **

Note that this PR is dependent upon PR #10 and should not be merged until #10 has been merged.

<b>Changes:</b>
-Group leaders can now accept/decline applications to their group
-The group-users (members) association is now present.  The number of members is also displayed and updated as expected.

<b>Tasks Remaining:</b>
-Provide button for members to leave a group
-Gray out/don't allow users to apply to a group more than once - a single user won't be counted more than once in a group, but multiple duplicate applications will be created with the current implementation
-Gray out/don't allow a user to apply to a group which they are a member of