Add API module for retrieving JSON Schema

This change adds a 'jsonschema' API module on wikis which enable the Schema
namespace. Like index.php?action=raw, the module returns the raw JSON content
of the schema. The rationale for having an API module is twofold:

 * It provides a way for looking up a schema by name *and* revision. action=raw
   ignores the title parameter when oldid is set, leading to bug 46174.

 * It is a step forward toward implementing JSON ref. The API module can be
   extended to accept an optional 'resolve' parameter which will return a fully
   dereferenced schema object. See bug 45886.

Bug: 46174
Bug: 45886
Change-Id: I4f17633726d0ac393bfe3dc2937738f50be38291
Merge "Add API module for retrieving JSON Schema"