Support for custom calendars in Wikidata dates

[wetneb]

Date values currently created by the Wikidata extension always use the Gregorian calendar. This should be configurable on a per-date basis. We just need to figure out a syntax for this.

Perhaps YYYY-MM-DD_QID, where QID is the item id of the calendar?

[wetneb]

For the changelog:

• Adds support for dates with custom calendars, such as 1345-03-07_Q1985786;
• Adds support for special date TODAY which evaluates to today's date when edits are made;
• Adds warnings for early dates using the Gregorian calendar.