This patch adds a transparent HTMLForm field that can be used to insert
the edit box inside an HTMLForm, and updates /test and /tools to use
that. The field class, together with the other editbox-related classes,
is now in a dedicated namespace. A future TODO is making it a real
HTMLForm field.
Also improve a bit the form in /test: add section labels and
avoid reusing the same label message used on Special:AbuseFilter.
Bug: T261584
Change-Id: Ib74bb5fdba4f8476169b754030fce6d4f72ce65a
