Allow blacklisting certain namespaces in Special:ShortPages

Allow blacklisting certain namespaces in Special:ShortPages

This new config variable ($wgShortPagesNamespaceBlacklist)
allows wikis to specify namespaces which should be excluded from
Special:ShortPages.

This will be used by Commons, for which NS_FILE is a content namespace
(accessible via Special:Random and Special:Nearby) but is not useful
to list on Special:ShortPages.

If the blacklist cancels out all namespace in wgContentNamespaces then
all namespaces will show up on the page.

Bug: T170687
Change-Id: I10b4849a5d7f3f8af75ccc6cfba230d03725c898