Investigate switching calendar providers

[MelissaAutumn]

Our current calendar provider Calendarific isn't really meeting the demand when it comes to supplying holiday calendars (See: https://github.com/thunderbird/thunderbird-website/issues?q=is%3Aissue+is%3Aopen+label%3A%22calendar+provider%22). At the same time we don't have the capacity to review and merge manual calendar PRs.

A better solution is pulling the data from a repository like https://github.com/commenthol/date-holidays which can be individually contributed to.

For any countries they don't support we can still mixin Calendarific data afaik.