ApiQuerySiteinfo: Add prop=restrictions for protection information

As pointed out on VPT, we previously had no way to find out about these.

This just dumps the raw config variables for now. They are documented
in DefaultSettings as having some magic applied to them when used,
maybe we should apply this magic here too, or in Setup.php?

https://en.wikipedia.org/w/index.php?title=Wikipedia:Village_pump_(technical)&oldid=594218538#API_query_to_list_available_protection_levels_for_a_wiki.3F

Change-Id: I4c1303f2a6fe6b60b44e960cc1253c555fc658dc
Merge "ApiQuerySiteinfo: Add prop=restrictions for protection information"