Create an InviteEmail worker.
Use InviteEmail worker in InvitationsController create action.
Merge pull request #4277 from davexunit/feature/4069

Background actual mailing when sending invitations
add changelog entry for #4277 [ci skip]
bumped unicorn timeout to 90 seconds and made it configurable
Merge remote-tracking branch 'upstream/develop' into pod.jpope.org