fix hardcoded limit on titles in Special:Export

fix hardcoded limit on titles in Special:Export

In Special:Export if you enter a category in the "Add pages from
category" textbox, there was a hardcoded limit of 5000 page titles in
the function getPagesFromCategory().

The same is true for a similar function fetching pages by namespace
instead of category, function getPagesFromNamespace().

I have a couple of wikis where we wish to export a nummber of pages
exceeding 5000, so this is inconvenient. In this commit, I have
introduced one new global configuration variable: $wgExportPagelistLimit.

This new configuration variable has had its default set in
includes/DefaultSettings.php to the values the two affected functions
were hardcoded to prior to this patch; 5000 in both instances.

This way, I can adjust the number of pages returned in the
Special:Export page by adjusting the above new variable in
LocalSettings.php.

Change-Id: I6ca9e26eb6bc4a7a2bafd73b9460f445940c8ecb