Add tests for gig calendar

When migrating moment to dayjs, there were no tests to check the calendar was showing the correct dates, so I had to test it manually. It would be useful if we had tests to check:

- First day of week is what the user specified in the customiser
- Dates line up with days shown at the top of the calendar (so in January 2021 the 1st would be in the column labelled "Fri")
- The year switching select box updates to show the 20 years around the selected year and has the correct month/year selected when the page loads (this was a problem in moment due to mutability that shouldn't reoccur but I failed to fix at the first attempt).