feat: Generate a partial schedule when there are more sessions than timeslots (ietf-tools#4950)

* refactor: move session/timeslot selection for sched editor to querysets

Purpose here is to make it easier to reuse the session and timeslot selection
logic between the schedule editor and the schedule generator. Additionally
resolves a todo-list item to unify the list of TimeSlotType ids in the
IGNORE_TIMESLOT_TYPES tuple and the SessionQuerySet.requests() method.

* refactor: use new helpers to select sessions/slots for sched generator

* refactor: eliminate some code lint

* feat: Split sched gen TimeSlot into scheduled/unscheduled variants

(work in progress)

* feat: First pass at supporting unscheduled timeslots

(work in progress)

* feat: Handle unscheduled timeslots in make_capacity_adjustments()

(work in progress)

* feat: Handle unscheduled timeslots in time-relation constraint check

(work in progress)

* feat: Reflect unsched timeslots in messages from by schedule generator

* fix: Prevent exception in pretty_print() if base schedule not assigned

* refactor: Avoid flood of time relation constraint warning messages

* test: update test_too_many_sessions

Author: jennifer-richards