Corrected handling of dateModified & datePublished

[tomschr]

Fix DOCTEAM#1367

This PR contains the following changes:

• Combine separate json-ld-dateModified and json-ld-datePublished into json-ld-dateModified-and-Published.
  Produce only an entry, if dates are valid and if datePublished < dateModified
  Use first revhistory/revision/date for dateModified and last revhistory/revision/date for datePublished.
• Revamp santize-date(date) template date can be any date in the format YYYY-M, YYYY-MM, YYYY-M-D, YYYY-MM-D, YYYY-M-DD, or YYYY-MM-DD. If the date doesn't conform to any of these formats, an empty string is returned.
• Add new validate-date(date) template $date should be in the format of YYYY-MM-DDTHH:MMZ, but only the YYYY-MM-DD is checked. If the date is valid, true() is return, otherwise false()
• Add compare-dates(date1, date2)
  Compare two dates lexicographically. Return -1 if date1 < date2; 0 if date1 == date2; 1 if date1 > date2.

---

In simple words, the stylesheet follows this approach:

1. Retrieve the possible date elements from <revhistory>.
2. Sanitize different formats into the consistent ISO format YYYY-MM-DDTHH:MMZ
3. Validate the dates
4. Compare the dates so it follows datePublished < dateModified
5. If this is all correct, create the respective JSON-LD structure. If there are invalid dates, skip it.