Email users when they are invited to groups

[mbramson]

Currently when a user is invited to a group, this happens essentially silently. They either have to go to their invitations explicitly, or if #162 has happened, be in the app and see a notification.

It would be great if we emailed users when this type of thing happened, preferably with a link to immediately accept the invitation (linking to the UserGroupMembershipRequestController's :update route if possible.