Allow units to be specified for quantity values.

Units are to be specified as full http(s) URIs (or "1").
Recommended vocabularies are QUDT or Wikidata.

This change does not touch on the issue of unit conversion or indexing.

NOTE: This is at odds with how QuantityValueParser parses units.
This means that it is not actually possible to enter a URI as a unit
in the quantity input widget's text field. So, until T77981 (and
perhaps T77982) are implemented, units can not actually be entered though
the UI. They can however be specified when setting values via the API.

Bug: T65722
Change-Id: Ib903325ac0dca57d49129c0e83e1e6e1211ecfa7
Avoid RegexValidator if possible

... and a bit of refactoring, mainly:
* Removing optional parameters.
* MembershipValidator uses strict comparison now.
* Commons file name check is a little bit more strict.

Change-Id: I85e908dfd8ae36a6c6bead024005fadb47837b21
Reduce non-functional validator nesting

... mainly removing composite validators that contain a single
validator only.

Change-Id: Ia42f7022d495bd89fa5ab407ffaed14147eee073
Merge "Allow units to be specified for quantity values."
Merge "Avoid RegexValidator if possible"
Merge "Reduce non-functional validator nesting"