Using $wgExtraNamespaces overrides any localized namespaces with the
canonical form, which is not ideal.
Namespaces added through extension.json will now store the canonical
form and numerical id in a 'ExtensionNamespaces' attribute that is read
by MWNamespace::getCanonicalNamespaces().
Also fix the documentation on $wgExtraNamespaces, as using
$wgCanonicalNamespaceNames has not been possible since r85327.
Bug: T110389
Change-Id: I5bd9a7258f59d8c4a7ad0543d2115960fbea9b3a
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
