RCFilters UI: Fix FilterTagItemWidget styles

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