Improve theming of tagcloud facets

The CSS classes should be inherited from the block level, i.e. remove "luceneapi-facet" class and make use of the "block-luceneapi_facet" class. Add a template should also be added with the goal of removing markup from the theme functions. The theme functions should also be renamed to luceneapi_tagcloud_facets() an luceneapi_tagcloud_facet() for consistency.