* We add the `.mw-interlanguage-selector` class to the
.vector-menu-heading in the server rendered HTML. `ext.uls.interface.js`
later attaches a click handler to this selector that loads the rest of
ULS.
* We hide the dropdown arrow for js users and only show it again if
ext.uls.interface module isn't installed or is not being loaded.
* When the `ext.uls.interface` module has been loaded, we hide the checkbox
and checkbox hack menu in favor of showing the ULS popover.
Additionally:
* Adds '.vector-menu-heading' class to menu headings.
* Change h3 selector to `.vector-menu-heading`.
Bug: T273232
Change-Id: I6f4572c16ca4096dcda3aac4d585003b93dcccfa