Enforce a .json extension (in DEV mode only).

Also changed the only example I found of it not doing so.

This is needed by some changes I am making to the routing.
Merge branch 'master' of github.com:guardian/frontend into the-json

Conflicts:
	dev-build/app/Global.scala
Update comment to reflect changes
Merge pull request #2143 from guardian/the-json

Enforce a .json extension (in DEV mode only).