(bug 44873) Add setting to exclude namespaces

- make it default to include all namespaces, but with option to exclude some
- it's insane to specify settings for all wikipedias to include namespaces;
much better to have wikibase for all namespaces by default, and we can exclude
some if requested.

Change-Id: I1a96321d43beb621d5a28ea024fb46477e0abafc