autocomplete: don't send cookies in publicly cached responses.

Fix session cookies being sent in publicly cached /autocomplete.json
responses. We can't set any cookies in a response that is being publicly
cached, otherwise they'll be visible to other users. If a user's session
cookies were to be cached, then it would allow their account to be stolen.

In reality, well-behaved caches like Cloudflare will simply refuse to
cache responses that contain cookies to avoid this scenario.

https://support.cloudflare.com/hc/en-us/articles/200172516-Understanding-Cloudflare-s-CDN:

    BYPASS is returned when enabling Origin Cache-Control. Cloudflare also
    sets BYPASS when your origin web server sends cookies in the response
    header.

test/functional/autocomplete_controller_test.rb

@@ -34,6 +34,14 @@ class AutocompleteControllerTest < ActionDispatch::IntegrationTest
         assert_autocomplete_equals(["rating:safe"], "rating:s", "tag_query")
         assert_autocomplete_equals(["rating:safe"], "-rating:s", "tag_query")
       end
+
+      should "not set session cookies when the response is publicly cached" do
+        get autocomplete_index_path(search: { query: "azur", type: "tag_query" }), as: :json
+
+        assert_response :success
+        assert_equal(true, response.cache_control[:public])
+        assert_equal({}, response.cookies)
+      end
     end
   end
 end