Only check W0 headers for m.wikipedia.org

Requires updated mwapi since we now use the API URL
to disable W0 header checking when we go to wikidata.org.

Split the patch I1e20a652a3e3b81301708ab60eeba416bd138af6
into two, since this needs to be merged asap as to not break the build.

Change-Id: I388ec6328b6127d54bc0bb84854cd949efdd732f