Full HTTP API for delegation

[martinthomson]

Again, for the combination of #31 and #32 and #38, if you want to do the full delegation without touching JavaScript...

This builds on #47 and adds an HTTP interaction for the delegation stage. The key here is that delegation can be done toward multiple different entities.

Again, this is triggered by any request that is made to the top-level site:

GET /top-level/pixel
Host: example.com

The response effectively executes the delegateConversion API (or whatever it ends up being called):

200 OK
Attribution-Delegation: all=the;parameters=needed
Link-Template: <https://intermediary1.example/track{?delegation_id}>;\
  rel=attribution-delegation
Link-Template: <[https://intermediary2.example/track](https://intermediary1.example/track){?delegation_id}>;\
  rel=attribution-delegation
Content-Type: whatever/the-request;needs

The browser recognizes the special new header and the new link relation types. It executes the attribution function according to the API and sends a request to each of the delegates. This uses Link-Template so that the sites can tell the browser where to stick the delegation ID.

Again intermediarySites is inferred for each delegation.