Category support for topic summary and header

Records categories via ParserOutput for topic summaries and
headers. Adds categories to OutputPage when performing 'view'
actions.

Currently has default output on category pages, there is a follow
up patch that requires a core change to enable the prettier
display.

Bug: T87793
Change-Id: I4f1433bd133bf4cdb94fda6bd0d7256f2eb98194