Consolidate API for page-less html2wt / wt2html

  GET /prefix/page -- works just as it does now. No wikitext passed in.
  POST /prefix/{page} -- with page optional.
    - if "wt" is provided, convert wt2html. "body" flag to return just
      the body
    - if "content" or "html" is provided, convert html2wt

  The _wikitext and _html form inputs now share the same backend.

Bug: 55758
Change-Id: I7ce70b98be802ba0894826629b1a21a083a4fde3
Merge "Consolidate API for page-less html2wt / wt2html"