Move experimental "v3" API to /api/experimental

Endpoints implemented with Rails Controllers will eventually be
removed, in favour of the Grape implementation added in ac2c89c0.

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Update prefix in ng PathHelper to experimental API

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Merge pull request #1472 from opf/feature/rename-experimental-api-prefix

Move experimental "v3" API to /api/experimental