WIP Commit for fixing up the linked days.
Proposed changed to allow multiple linked days.

This is a tentative change since the implications of this change
affect a wide-array of things in the application.

Reports rely on Linked Days, Various displays rely on it, calculations
etc. The proposed change has included a boatload of tests to allow try
to ensure that we don't step on any of the meaninful toes, the rest
can be discovered as and when. This sounds cavalier but I am not a
psychic.
Removed migrations.
Added the real migration code for moving tracker.models.TrackingEntry.link from a OneToManyField to a ForeignKey.
We referenced a model's field, which we have changed, so change the comments to reflect that.
Re-enable displaying linked date entries in the calendar.
Retain the link day when changing entries, this didn't come up before since the work/link days weren't allowed to be changed.
When denying and entry, we just unlink, this will smartly choose whether to delete the entry based on how many other entries are linked to this entry.
Enabled the change_link datepicker in the calendar when picking an entry.
Whitespace.
Reworked the ajax_change_entry logic to accomodate linked entries and multiple links.
When deleting the FK relationship in the linked entries don't cascade the deletes to the related object.
Reflowed the logic for displaying the error message in the calendar page.
In the mec_ot_report just ignore any entry which is linked.
Reworked the ajax_change_entry so it accurately reflects the rules around not being able to add an undertime entry with a linked day.
Remove initial migration.