De-normalize project visibility_level in issues in Elasticsearch

This is necessary to search issues correctly without joining to the
parent project. The previous case where we only set
`issues_access_level` will not correctly handle "Internal" projects
because "Internal" does not change the `issues_access_level` of the
project and we need a way to ensure that issues from "Internal" projects
are only found by logged in users.

ee/lib/elastic/latest/issue_instance_proxy.rb

@@ -13,6 +13,7 @@ module Elastic
         end
 
         data['assignee_id'] = safely_read_attribute_for_elasticsearch(:assignee_ids)
+        data['visibility_level'] = target.project.visibility_level
 
         # protect against missing project_feature and set visibility to PRIVATE
         # if the project_feature is missing on a project

ee/spec/models/concerns/elastic/issue_spec.rb

@@ -107,6 +107,7 @@ RSpec.describe Issue, :elastic do
 
   it "returns json with all needed elements" do
     assignee = create(:user)
+    project = create(:project, :internal)
     issue = create :issue, project: project, assignees: [assignee]
 
     expected_hash = issue.attributes.extract!(
@@ -129,7 +130,8 @@ RSpec.describe Issue, :elastic do
     })
 
     expected_hash['assignee_id'] = [assignee.id]
-    expected_hash['issues_access_level'] = issue.project.project_feature.issues_access_level
+    expected_hash['issues_access_level'] = ProjectFeature::ENABLED
+    expected_hash['visibility_level'] = Gitlab::VisibilityLevel::INTERNAL
 
     expect(issue.__elasticsearch__.as_indexed_json).to eq(expected_hash)
   end