(bug 44576) Trigger redirect based on content negot.

This implements content negotiation for access to entity data.

If no serialization format has been given explicitly, a format
is picked automatically based on the Accept header, and the client
is forwarded to the canonical URL for that format.

This allowes for Special:EntityData to back a rewrite rule that
implements the canonical entity URIs,
e.g. http://www.wikidata.org/entity/Q2013

Change-Id: I2f0c110de7d606e44cffa9600a3dfb3b160f9281
Merge "(bug 44576) Trigger redirect based on content negot."