RCFilters UI: In the highlight code, use .data() the way it was meant to be used

RCFilters UI: In the highlight code, use .data() the way it was meant to be used

Use .data( 'highlightedFilters' ) instead of .attr( 'data-highlightedFilters' ),
and make the value an array rather than a string that we have to convert
back and forth every time. We have to use .attr() for data-color because it's
used by CSS, but data-highlightedFilters isn't used by anything else.

Using an array makes the code much simpler. Also add the class
mw-rcfilters-highlighted to every highlighted item, so we can find
them back easily and don't have to use $( '[data-highlightedFilters]' )
(attribute selectors are slow, and this wouldn't work with .data() anyway).

Bonus: use the comma-separator message

Change-Id: I04de7f8fb74d60fb23ef47bf50bacfeb176a55b1