Ready for Review/Merge - Group join/leave

<b>Note</b> that this PR is dependent upon PR #10 and should not be merged until #10 has been merged (see similarity in commit history).  I'll rebase and cleanup the history before marking as ready for review/merge.

edit: #10 is now merged and I have rebased this branch so that it is up to date with master (and only displays the new commits/changes)

<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.
-Button for members to leave a group (on group info page)
-Don't allow a user to apply to a group which they are a member of ('apply' button isn't visible)
-Ability for group leaders to destroy the group
-Apply button is grayed out if the current user has a pending application
-Fixed styling issue where buttons were taking up an entire line (wasn't visible before simply because there was only one button)