WIP: Adjust hiding of overlapping public holidays and absences

[sliverc]

Determine by location of employment at the date of absence whether a public holiday overlaps it.

Following fix needs to be released before this works: https://code.djangoproject.com/ticket/28442

Hence before this can be merged Django needs to be updated to at least 2.1.

[velrest]

@trowik Is this still an issue or has this been implemented already?

[trowik]

@velrest this hasn't been implemented