Add support for dates before 1970-01-01, especially for historic dates and times

[nuest]

• for old dates probably easier to just have authors enter the date
• if the author does not enter MM-DD then the calendar should not add it - maybe enable the calendar popup with a button click?
• in PHP
  • https://carbon.nesbot.com/
  • https://stackoverflow.com/questions/31069210/php-handling-dates-before-and-after-christ (1000-01-01 ... only)
  • https://scholarslab.lib.virginia.edu/blog/parsing-bc-dates-with-javascript/

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TODO: Ask on humanities RSE lists about this - they surely must know!

[nuest]

Current thinking: just store date as text string, e.g., 1000-01-02 or 2020. This would also allow to be more flexible with the dates. Then we just return the text and use that in JavaScript.

[nuest]

• Begin and End as separate fields, for 3.3 both fields are required, Note "begin and end can be the same date"
• Allow any number or simple date (YYYY-MM-DD), but also partial dates: YYYY, -YYYYYYY, YYYY-MM
• no changes in the database storage
• DateRangePicker is removed
• nothing for validation
• presentation stays the same (plain numbers)

3.4 stuff > #140