Make the 'interwiki' option temporarily accept null as well as true/false
(and default to null instead of false) so form fields which disallow
interwiki titles can avoid deprecation warnings. This also includes
implementing the new error message.
The idea is that for the duration of the deprecation process, form fields
can be set explicitly to interwiki => false (which will eventually become
the default). It would be super annoying to have to do this for all
title fields, but this way it's only needed for fields where users actually
submit interwiki titles with some frequency.
Also improve the logic for legacy fields (which do not set the option
either way): since these (while emitting a warning) allow an interwiki
title, it does not make sense to apply namespace etc. checks to that
title.
Bug: T288155
Change-Id: Ic00f4a0f27747b5ff0893b4c01f42f68a99771ab
