Enhance Special:EnableFlow to allow enabling Flow on a talk page that already exists.

• If the page already exists, the Special page
  • it will move it to a default archive subpage location (based on T93395: E3. Allow config of archive title formats using MediaWiki namespace config page), https://trello.com/c/5hVDK1XP.
  • (Should it auto-protect the archive page? What if the namespace doesn't support subpages?)
  • it will add a link to the archive to the header, https://trello.com/c/l48SFDNU
  • There should be anoption to put the wikitext of existing page that's above the first heading ("section 0") into the new Flow board header

Those last two should already be implemented by the Import code.

It needs to report in detail any problems it has, e.g. "Enabled Flow on Talk: *Some Page* but did not archive existing wikitext content (failure *Xxxx*)"

Issues

• How do we deal with Redirects? (Page-Moving an active Flow board will need to leave redirects behind.) (Should work the same way Import code does).

Broken out from T72073: U4. Special page to enable Flow on existing wikitext talk pages, auto-archiving the wikitext page (v2).