Add issue link into issue due notification email

- plus a test.

app/views/notify/issue_due_email.html.haml

 %p.details
-  #{link_to @issue.author_name, user_url(@issue.author)}'s issue is due soon.
+  #{link_to @issue.author_name, user_url(@issue.author)}'s issue #{issue_reference_link(@issue)} is due soon.
 
 - if @issue.assignees.any?
   %p

app/views/notify/issue_due_email.text.erb

 The following issue is due on <%= @issue.due_date %>:
 
-Issue <%= @issue.iid %>: <%= url_for(project_issue_url(@issue.project, @issue)) %>
+Issue <%= @issue.iid %>: <%= issue_reference_link(@issue) %>
 Author:    <%= @issue.author_name %>
 <%= assignees_label(@issue) %>
 

changelogs/unreleased/cablett-issue-due.yml

+---
+title: Add issue link to due date emails
+merge_request: 50642
+author:
+type: changed

spec/mailers/notify_spec.rb

@@ -224,6 +224,10 @@ RSpec.describe Notify do
           let(:model) { issue }
         end
 
+        it 'contains a link to the issue' do
+          is_expected.to have_body_text(issue.to_reference(full: false))
+        end
+
         it_behaves_like 'it should show Gmail Actions View Issue link'
         it_behaves_like 'an unsubscribeable thread'
         it_behaves_like 'appearance header and footer enabled'