RCFilters UI: Fix FilterTagItemWidget styles
The styles got a bit messed up when we switched to using
TagMultiselectWidget. Fixes:
* Switch to using the methods of flaggable element (which TagItemWidget
already is using) instead of custom css classes
* Use flag:invalid for conflicts, so the styles are consistent with
OOUI styling for invalid tags, but keep the label red color, so it
is still evident when the tag is selected.
* Bonus: Actually apply filterTagMultiselectWidget base class, because
it wasn't applied before (oops)
Bug: T163522
Change-Id: I1cc3bd743d8b930bee23386516ca123ee3c36983