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.

[petercpg]

Hi @MelissaAutumn, it's zh-TW locale owner (volunteer) and Thunderbird localizer here, I also contributed to few versions of the old Sunbird/Lightning caldata on bedrock before.

I just came to this topic via #646. Not sure if we would reach out to product l10n communities to prepare these holiday calendar files?

That is, if a locale is able to manage the calendar files, use that file; if not, then make use of some library or external API services.

For Taiwan's holidays, Calendarific is problematic, there are few entries on https://github.com/commenthol/date-holidays/blob/master/data/countries/TW.yaml is still in Simplified Chinese but their YAML syntax format should work.