Add triggers to prevent cross-parentage

[jace]

Some models share Project as a parent but also have foreign key relationships to each other. The Schedule model, for instance, links to Project, Proposal and VenueRoom and the latter two also link to Project. It is possible for the Project to be different between these various rows, which is (a) a violation of third normal form, but accepted here as a necessary trade-off, but (b) also a violation of the application data model.

Funnel should have post-insert and post-update triggers on such models to check for shared parentage of affiliated models. Code for this can be adapted from Coaster's add_primary_relationship trigger.

[jace]

We're moving away from cross-parentage design in all current work. Venue and VenueRoom remain as legacy models that should be addressed when offline events pick up again, post-pandemic. Closing this ticket.