Hide edit toolbar Signature button in non-discussion namespaces

Hide edit toolbar Signature button in non-discussion namespaces

Most wikis only use user signatures on pages set aside from discussion,
(Talk namespaces and sometimes project/main namespaces depending on
the wiki), so having the button available everywhere is confusing.
The few wikis that need the button (especially non-content,
internal/corporate/planning wikis), can add relevant namespaces to the
$wgExtraSignatureNamespaces array in LocalSettings.

This would make it possible to solve bugs like T59727 or T53154.

Since this is a change to default behavior, a release note is added.

Bug: T7645
Change-Id: I7ccf1093b888c7b33721234349ca0ac054c3cd3f