Web API: make next link contain full URLs

For pagination purposes, the next links or our Web API look like this:

Link: </api/1/origin/1/visits/?last_visit=2&per_page=2>; rel="next",

while standard conformant, that is different from what GitHub/GitLab do, which have complete URLs starting with the scheme:

Link: <https://api.github.com/search/code?q=addClass+user%3Amozilla&page=2>; rel="next",
Link: <https://gitlab.example.com/api/v4/projects/8/issues/8/notes?page=3&per_page=3>; rel="next", 

Also, our way is more annoying for clients to exploit, as it requires some URL gymnastic.

We should change our next links to contain full URLs too.

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Migrated from T2147 (view on Phabricator)