Bug 67249: allow filtering of newpages by ns

Partial workaround:
If namespace/ns parameters are specified before -newpages, provided
namespace values are used, otherwise default is ns=0.

Same logic applied to RecentChangesPageGenerator to improve efficiency
of the query, if wanted.

Change-Id: I20a4b04bb8f30cd750d43856a89cdf471898f84e
Merge "Bug 67249: allow filtering of newpages by ns"