Support host entity start/end date

Problem/Motivation

As it says in the docs for registration_scheduled_actions:

Many sites will have a date field on their events that is separate from the registration settings close date.

Perhaps the host entity should have a getStartDate() and getEndDate() method (default NULL). And the registration field settings should allow specifying another field on the entity to use as the source for these.

There may be various places in the logic of the module where it makes sense to make use of these if they are available. For example, perhaps registering for an event after its end date should be impossible, even if no close date is specified.

Remaining tasks

User interface changes

API changes

Data model changes