Improve registration/documentation display in templates

Several changes to the event detail, event list, and event archive
views:

* Students can still see cancellation forms even when events are set to
prevent registration (closes #19).

* We suppress the "Registration is closed" indicator for past events.

* We hide registration forms when events are set to "hide forms". Users
see no information about registration at all, which is by design: this
setting is intended for outside events or ones that don't require
registration. That should be made clear in copy.

* On the event detail page, we provide a little more visual indication
that a student has registered.

* Past events have a "Past" registration indicator on their detail page.

* Events that have documentation show a camera icon with a "See
documentation" indicator.

* We show documentation for events that have some.