This sets up the tags:
* discussiontools
* discussiontools-reply
* discussiontools-edit (not yet implemented)
* discussiontools-newsection (not yet implemented)
The tags are flagged as user-addable, because otherwise they can't be
passed through to the VE API (at least, not without editing it so that
it explicitly knows about them, which seems like a strange
interdependency). It's assumed that letting users who know about the
tags add them to random changes via action=editchangetags would be
(a) the pettiest and most inconsequential vandalism possible, and
(b) unlikely to happen.
This relies upon I2c1d0f8d69bc03e5c1877c790247e165f160e966 in
VisualEditor to not also tag the edits with `visualeditor`.
Bug: T242184
Change-Id: I4e5e26afdd52279df242e1912f073b415b812c3b
664b2890d7