RCFilters UI: Fix FilterTagItemWidget styles
The styles got a bit messed up when we switched to using
- 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)