Load 'ext.visualEditor.mwsignature' (which implements VE's existing
handling for signatures), then subclass and override a bunch of things
in order to:
* Replace the context menu with a note that you don't need to type the
signature when commenting using the reply widget
* Override the sequence/command to insert signature so that it selects
it afterwards and thus displays the context menu
* Treat signatures as signature nodes when switching from wikitext,
instead of the normal pre-save transform turning them into regular
links and text
Bug: T255738
Change-Id: Icb542451c2307ab51e56bd627804096c7b5552c8
