Add Flow MessagePoster


Add Flow MessagePoster

This will allow mediawiki.feedback to post to Flow boards,
as well as the same for gadgets and user scripts.
They do not need to know ahead of time whether it is a regular
talk page or Flow board.

This requires the MessagePoster class added in
Ic69acafb24aa737536fe3a074e1958690732f0a7 .

Bug: T91805
Change-Id: Ic817de4178a912530fd26a94c5904bb5b0f87881
(cherry picked from commit 8a26cfd40b8ae38721b8065414009b7af83a6d5a)