Refactor directive IDs in the scheduler

[JoelCourtney]

• Tickets addressed: closes Refactor directive ids in scheduler #1505
• Review: By commit
• Merge strategy: Merge (no squash)

Description

This contains a couple behavioral changes:

• All activity directive id mapping or transforming has been removed, except for the upload step where directive ids might not match what they ultimately become in the database.
• Generated activities are no longer assigned fake directive IDs. They are not directives, so this was unsound.

And several renames / trivial refactors:

• Renamed SimulatedActivity[Id] to ActivityInstance[Id] to better contrast with ActivityDirective.
• Deleted SchedulingActivityDirectiveId and replaced all usages with ActivityDirectiveId.
• Renamed SchedulingActivityDirective to SchedulingActivity because it also holds instance information.
• Renamed MerlinService and its implementors to MerlinDatabaseService because merlin is not the database.

Verification

All tests still pass. Please suggest any new tests you think would be helpful.

Documentation

I don't think we had any internal documentation of the ID maps, which is part of what made them so annoying. So none needs to be updated!

Future work

[mattdailis]

A lot of very sensible changes here. Looks good to me based on visual inspection - if you can get the tests pass I'd say ship it