Adapted accept invitation form to the styles used in Feedbunch for devise views.
Finishing touches for the invitation feature.

Fixed error in the URL for the accept invitation form.

Removed the "destroy invitation" route. Users won't be able to destroy invitations; rather, unaccepted invitations will be destroyed automatically after a certain amount of time (this is not yet implemented).

Only the create invitation action is handled by our customized FriendsInvitationsController. The rest of actions are handled by the default devise_invitable controller.