Add a "Holidays" calendar based on the country

Similar to #397

## Prerequisites
- [x] I have searched open and closed issues for duplicates.

## Feature
Like most calendars apps out there, it would be nice to have a default "Holidays" calendar showing all the holidays of the user's selected country.

**Existing work**
Thunderbird provides many .ics (as well as online subscriptions) calendars for a list of selected countries, maybe we can use them.

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