Link: rel="alternate" to JSON for queries too

[simonw]

Following:

• Add Link: rel="alternate" header pointing to JSON for a table/query #1533

I implemented it for tables and rows but I should have done queries as well.

[simonw]

I just realized I can refactor this to make it much simpler.

[simonw]

% curl -s -I 'https://latest.datasette.io/' | grep link
link: https://latest.datasette.io/.json; rel="alternate"; type="application/json+datasette"
% curl -s -I 'https://latest.datasette.io/fixtures' | grep link
link: https://latest.datasette.io/fixtures.json; rel="alternate"; type="application/json+datasette"
% curl -s -I 'https://latest.datasette.io/fixtures?sql=select+1' | grep link
link: https://latest.datasette.io/fixtures.json?sql=select+1; rel="alternate"; type="application/json+datasette"
% curl -s -I 'https://latest.datasette.io/-/plugins' | grep link 
link: https://latest.datasette.io/-/plugins.json; rel="alternate"; type="application/json+datasette"

[simonw]

I ran the following on https://www.google.com/ in the console to demonstrate that these work as intended:

[
  "https://latest.datasette.io/fixtures",
  "https://latest.datasette.io/fixtures?sql=select+1",
  "https://latest.datasette.io/fixtures/facetable"
].forEach(async (url) => {
  response = await fetch(url, {method: "HEAD"});
  console.log(response.headers.get("Link"));
});