Allow group admins to manage invitations

[mbramson]

Currently only group owners can manage the invitations for a group. Group admins should be able to view, create, accept, and reject invitations on behalf of the group that they are an admin of. This will involve giving group admins access to all actions of the GroupInvitationsController.

Group admins were implemented in #150 and are defined as a user whose UserGroup association with the group they are an admin of has an is_admin property of true.

You will probably want to use the Ecto query defined by Pairmotron.UserGroup.user_group_for_user_and_group(group_id) to determine if the current user is the group admin for the current group.

In addition, group admins should be able to see the "invitations" link on the group/group_actions.html.eex template that group owners can see.

This is what they currently see:

Here's the button that they need to be able to see: