INTL-364 Use holidays gem

This gem previously contained information for UK public holidays in order to calculate working days.

For International, we need similar functionality for other countries, so rather than maintain all this ourselves I have chosen to use the [holidays](https://github.com/alexdunae/holidays) gem.

Lots of code has been removed, what remains is a fairly thin wrapped/adaptor around the holidays gem, preserving the previous interface so that no changes to the main NOTHS app are required.

I noticed a couple of mistakes in the holidays gem, so I'm using a fork until these are fixed:

* https://github.com/alexdunae/holidays/pull/50
* https://github.com/alexdunae/holidays/pull/51

TODO:
- [ ] squash after +R

In followup PRs I will:
* rename gem to `working days`
* add tests for DE
* provide configuration option