Implements HTMLTagMultiselectField, a form field that instantiates
TagMultiselectWidget, the PHP representation of the OOUI's js-based
widget of the same name with the implemented parameters:
* allowArbitrary
* allowedValues
Bug: T278317
Change-Id: I3a6a30506d493be4185f917c577b3837fffd8ae1
Explicitly declare all dynamic properties. Add docblocks for several
props. Remove the defaults when they're already set in the constructor.
Make TagMultiselectWidget use ?? like other widgets.
Change-Id: I889bf6f9e1bbe381f5bffb8ce1370c9e2e863520
TitlesMultiselectWidget and UsersMultiselectWidget share
a lot of functionality, so implement a common base class.
This also adds some things to UsersMultiselectWidget:
* shows a pending element to users with JavaScript
* makes the input configurable
Change-Id: Ie6649b476c64e795254f457e3863fa7f14aa05ac