Multiple Reference Timelines (Beta 2)

[purfectliterature]

Continue the work merged in #5491 and #5631. These enhancements are to be released in the next beta cadence.

• Fix Workbin (Assessment) times not updating when changing times in Timeline Designer (reparent folders)
• Fix ClosingReminderJob not being re-set up when assessment's time is changed (see Course::ClosingReminderConcern)
  → Reset closing reminders when end_at is changed from a reference time #6005
• Make closing reminder emails read from personal times #3240 + custom reference times
• Check ActiveRecord queries that are still using default_reference_time
  Multiple Reference Timelines: Back-end models, controllers extensions #5491 (review)
• Reconcile modifications to FOMO personalisation strategy
  Multiple Reference Timelines: Back-end models, controllers extensions #5491 (comment)
• Fix TimePopup's Save button not collapsing in staging/production builds
• Fix TimelinesOverviewItem's custom times label singular/plural translation
• Make a new TimeBar that supports bonus end time > end time
• Sort day view by start time
• Support duplicate timeline
• Support offset timeline by some time duration
• Add left/right buttons to DayCalendar header for navigation when horizontal scrolling is not available