Improve UI for page protection with $wgNamespaceProtection

Right now, if you set $wgNamespaceProtection, the protection interface
is confusing: it will allow you to apply "useless" protection levels for
any namespace except MediaWiki, where it will refuse to let you set any
protection at all.

The fix is to find which restriction levels are more restrictive than
the $wgNamespaceProtection restriction (i.e. where there is at least one
group that can pass $wgNamespaceProtection but not the level from
$wgRestrictionLevels), and use only those in the protection form. If
there are no such levels, we can skip showing the "protect" tab
entirely.

Change-Id: I9e2b29ade566abcd008ea2ad1e2f9818e315bb32