Fix N+1 Gitaly calls in /api/v4/projects/:id/issues

This is a follow-up from
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31938.

In GitLab 9.0,
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9661 removed the
`subscribed` flag from the API when the user requested a list of issues
or merge requests since calculating this value triggers extensive
Markdown processing.

In GitLab 12.0 via a4fbf39e, we accidentally reintroduced this
performance regression by changing `IssueBasic` to `Issue` in
`entities.rb`. This showed up as a Gitaly N+1 issue since the Markdown
processing would attempt to extract a commit if it detected a regex that
matched a commit.

We restore the prior behavior by once again removing the `subscribed`
flag for the bulk list of issues and merge requests and add a test to
ensure they aren't reintroduced.

Relates to https://gitlab.com/gitlab-org/gitlab-ce/issues/66202

changelogs/unreleased/sh-fix-nplusone-issues.yml

+---
+title: Fix N+1 Gitaly calls in /api/v4/projects/:id/issues
+merge_request: 32171
+author:
+type: performance

doc/api/issues.md

@@ -284,7 +284,6 @@ Example response:
          "award_emoji":"http://example.com/api/v4/projects/4/issues/41/award_emoji",
          "project":"http://example.com/api/v4/projects/4"
       },
-      "subscribed": false,
       "task_completion_status":{
          "count":0,
          "completed_count":0

lib/api/issues.rb

@@ -163,7 +163,8 @@ module API
           with_labels_details: declared_params[:with_labels_details],
           current_user: current_user,
           project: user_project,
-          issuable_metadata: issuable_meta_data(issues, 'Issue', current_user)
+          issuable_metadata: issuable_meta_data(issues, 'Issue', current_user),
+          include_subscribed: false
         }
 
         present issues, options

spec/requests/api/issues/get_project_issues_spec.rb

@@ -446,6 +446,14 @@ describe API::Issues do
       expect_paginated_array_response([closed_issue.id, confidential_issue.id, issue.id])
     end
 
+    it 'exposes known attributes' do
+      get api("#{base_url}/issues", user)
+
+      expect(response).to have_gitlab_http_status(200)
+      expect(json_response.last.keys).to include(*%w(id iid project_id title description))
+      expect(json_response.last).not_to have_key('subscribed')
+    end
+
     context 'issues_statistics' do
       context 'no state is treated as all state' do
         let(:params) { {} }