Bug 52052: Add namespace configuration variable for navbar

By default, the navigation bars will be added to pages that are
in $wgContentNamespaces, but this allows the wiki admin to set
the namespaces in which the navbars should appear.

Bug: 52052
Change-Id: Ied6a1b9e644b9cf8d5109e7bf385ce8576abc634
Merge "Bug 52052: Add namespace configuration variable for navbar"