Add JavaScript variable wgContentNamespaces

Intended to be used in WikiEditor to hide the signature button in content
namespaces. Other uses could be for gadgets or extensions that only need
to work on content pages, or that are internally-facing and should only
be seen on non-content pages.

It is an array of namespace IDs, so that end-users can quickly check
whether wgNamespaceNumber is in this array.

Change-Id: I01d9671dd14aa79e79ed887299c2f5de766f7375
Merge "Add JavaScript variable wgContentNamespaces"