RCFilters: Move aggregation of highlight classes to the backend

RCFilters: Move aggregation of highlight classes to the backend

Construction of the highlight containers was moved to the backend, but
setupHighlightContainers() also aggregates the CSS classes used for
highlights for grouped entries in enhanced mode.

Move that to the backend too, and get rid of setupHighlightContainers().

Also move the namespace classes to getHTMLClassesForFilters() so that
they get picked up too, and pick up classes for tags separately because
the way they're handled is weird.

Bug: T197168
Change-Id: I4c374f82e7d128025f4e2b2f39b0adba14b76ef3