Opening/Closing reminder emails for multiple reference timelines

[cysjonathan]

Issue

Current database structure only supports on closing reminder email on the LessonPlanItem level, which means only one opening/closing reminder email can be sent per-lesson plan item (e.g. Assessment, Survey).

Expected behaviour

When multiple reference timelines is enabled, it's possible to have different groups of students having different closing times (e.g. group A closes on Monday, group B closes on Tuesday). They should similarly have different reminder emails sent out to each group (e.g. Group A should receive their closing reminder on Monday morning 0000h, and Group B should receive their closing reminder on Tuesday morning 0000h.)

Same goes for opening time.

Fix

Opening/Closing reminder email scheduled job creation should be aware of multiple reference timelines (i.e. the update of an individual reference timeline update should trigger a new scheduled job that invalidates the old job associated with the same reference timeline) If there are 3 reference timelines, then there should be 3 closing reminder email jobs.