- 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 <firstname.lastname@example.org>
Co-Authored-By: Bartosz Dziewoński <email@example.com>