Project RSVP box should remain post-event

[jace]

The registration box on a project is currently not shown when there's no schedule. This removes the option for a participant to cancel their registration, should they want to stop receiving updates.

The registration box should always be shown, and registration/unregistration should always be possible. The main change will be that the "Register" text will revert to "Follow" as is used for projects without a schedule.

When RSVP is disabled, any existing participants will still get the box, giving them the option to unregister.