Include auto-discovery header on REST responses

[danielbachhuber]

Adds Link header to REST API responses:

Link: <http://wordpress-develop.dev/wp-json/>; rel="https://api.w.org/"

When a client is attempting to auto-discover the API with a HEAD request, WordPress should always include the Link header to orient the client.

Header should be included on admin responses too

Punting this. Too late for 4.5 beta.

@rmccue Can you land this pretty please?

@danielbachhuber Picking this up to try and land in 4.6

Added unit test

In 35580.2.diff​ I added a unit test to check the presence of the Link header. This looks good to me and unless @rmccue objects I can land this.

@rachelbaker Ryan mentioned adding the header to /wp-admin/ requests too, which makes sense to me. Want to add that as well? Probably could happen in two separate commits.

@danielbachhuber Including the link header for wp-admin requests could probably be done using rest_output_link_header, but I don't feel strongly this is needed (rest_output_link_header already is hooked into template_redirect also included in dispatched API requests). Of course @rmccue can add if he likes.

In 37903:

REST API: Include auto-discovery Link header when serving API requests.

The Link header allows clients to verify if a site has made the REST API available, as well as indicating how to access it.

Props danielbachhuber.
Fixes #35580.