Allow relate quick action on issue create

Allow the use of the `/relate` quick action in the issue description
when the issue is first created. This enables users to use this feature
in issue templates.

Changelog: added

lib/gitlab/quick_actions/relate_actions.rb

@@ -17,11 +17,17 @@ module Gitlab
         params '#issue'
         types Issue
         condition do
-          quick_action_target.persisted? &&
-            current_user.can?(:"update_#{quick_action_target.to_ability_name}", quick_action_target)
+          current_user.can?(:"update_#{quick_action_target.to_ability_name}", quick_action_target)
         end
-        command :relate do |related_param|
-          IssueLinks::CreateService.new(quick_action_target, current_user, { issuable_references: [related_param] }).execute
+        command :relate do |related_reference|
+          service = IssueLinks::CreateService.new(quick_action_target, current_user, { issuable_references: [related_reference] })
+          create_issue_link = proc { service.execute }
+
+          if quick_action_target.persisted?
+            create_issue_link.call
+          else
+            quick_action_target.run_after_commit(&create_issue_link)
+          end
         end
       end
     end

spec/services/quick_actions/interpret_service_spec.rb

@@ -1935,6 +1935,21 @@ RSpec.describe QuickActions::InterpretService do
           it_behaves_like 'relate command'
         end
 
+        context 'when quick action target is unpersisted' do
+          let(:issue) { build(:issue, project: project) }
+          let(:other_issue) { create(:issue, project: project) }
+          let(:issues_related) { [other_issue] }
+          let(:content) { "/relate #{other_issue.to_reference}" }
+
+          it 'relates the issues after the issue is persisted' do
+            service.execute(content, issue)
+
+            issue.save!
+
+            expect(IssueLink.where(source: issue).map(&:target)).to match_array(issues_related)
+          end
+        end
+
         context 'empty relate command' do
           let(:issues_related) { [] }
           let(:content) { '/relate' }