mw.htmlform: Fields hidden with 'hide-if' should be disabled

mw.htmlform: Fields hidden with 'hide-if' should be disabled

Otherwise, in browsers that support HTML5 form validation, it
would be impossible to submit the form if a field failing
validation was hidden (e.g. because it's marked as required).

Note that disabled fields are not submitted with the form. This
should be okay, as server-side validation ignores them entirely.
(I37e50799ba1f0e0e64a197818b58444f5b056bf0 fixes a corner case.)

For cloner fields, 'hide-if' rules of the parent are now copied
to the children, to make handling such nested fields simpler.

Bug: T145440
Change-Id: I81d04dca6cbb499a15828fd33b01746b68c694da