Implement NamespaceInputWidget

Implement NamespaceInputWidget

* Add PHP version of NamespaceInputWidget, co-authored by Florian,
  which consists of a DropdownInputWidget offering a choice of
  namespaces and two CheckboxInputWidgets allowing to also match
  associated namespace (talk/content) or to invert the choice.
* Add an incomplete JS version of NamespaceInputWidget, which is only
  really functional when infused from the PHP version (it can't
  generate the dropdown by itself, for example). Implement some JS to
  improve the experience of selecting the "all namespaces" option in
  the dropdown (by disabling the checkboxes when this happens).
* Split off a 'mediawiki.widgets.styles' module, which has the basic
  styles for PHP widgets which are to be loaded in the head. Make
  OutputPage::enableOOUI() also add this module (which should stay
  reasonably small).
* Use the new widget in HTMLForm's HTMLSelectNamespace field. It can
  be seen in action on Special:LinkSearch, for example.

Co-Authored-By: Florian <florian.schmidt.welzow@t-online.de>
Co-Authored-By: Bartosz Dziewoński <matma.rex@gmail.com>
Change-Id: I5cbfa9d0f6a8641148ce476b7dbe65e9096b4485