RCFilters: add namespace filters for all contents and all discussions

RCFilters: add namespace filters for all contents and all discussions

These filters provide a shortcut to see or highlight all entries associated
with a subject or a talk namespace.

They are defined as real filters on the client-side and are superset
of all other namespace filters.

They are expanded to the list of namespaces they represent on the server-side.

Bug: T201520
Change-Id: Id9e04a38c2d7c8165c27af98959aa5715225faa8