feat: turn on schedule creation by default

This commit removes several waffle toggles that have been enabled
on edx.org for years. It's time to remove the rollout gating for
these features and enable them by default.

This doesn't directly change any behavior. But it does create new
database objects by default now and allows for enabling other
schedule based features more easily.

Specifically, the following toggles were affected.

schedules.create_schedules_for_course
- Waffle flag removed as always-enabled
- We now always create a schedule when an enrollment is created

schedules.send_updates_for_course
- Waffle flag removed as always-enabled
- Course update emails are sent as long as the ScheduleConfig
  allows it.
- This is not a change in default behavior, because ScheduleConfig
  is off by default.

dynamic_pacing.studio_course_update
- Waffle switch removed as always-enabled
- Course teams can now always edit course updates directly in Studio

ScheduleConfig.create_schedules
ScheduleConfig.hold_back_ratio
- Model fields for rolling out the schedules feature
- Schedules are now always created
- This commit only removes references to these fields, they still
  exist in the database. A future commit will remove them entirely

This commit also adds a new has_highlights field to CourseOverview.
This is used to cache whether a course has highlights, used to
decide which course update email behavior they get. Previously every
enrollment had to dig into the modulestore to determine that.