Implement tracking of time slots along with editing and restoration of
singularly edited time slots. This provides a simpler interface to work
with when rendering tables of time slots that can be edited than
directly manipulating events and shifts containing an array of slots.
Rename and refactor the types passed over the API to be based on an
entity that's either living or a tombstone. A living entity has a
deleted property that's either undefined or false, while a tombstone
has a deleted property set to true. All entities have a numeric id
and an updatedAt timestamp.
To sync entities, an array of replacements are passed around. Living
entities are replaced with tombstones when they're deleted. And
tombstones are replaced with living entities when restored.
