Add link to Merge request references in emails

app/views/notify/closed_merge_request_email.html.haml

 %p
-  Merge Request #{@merge_request.to_reference} was closed by #{sanitize_name(@updated_by.name)}
+  Merge Request #{link_to(@merge_request.to_reference, project_merge_request_url(@merge_request.target_project, @merge_request))} was closed by #{sanitize_name(@updated_by.name)}
+

app/views/notify/merge_request_status_email.html.haml

 %p
-  Merge Request #{@merge_request.to_reference} was #{@mr_status} by #{sanitize_name(@updated_by.name)}
+  Merge Request #{link_to(@merge_request.to_reference, project_merge_request_url(@merge_request.target_project, @merge_request))} was #{@mr_status} by #{sanitize_name(@updated_by.name)}

app/views/notify/new_mention_in_merge_request_email.html.haml

 %p
-  You have been mentioned in Merge Request #{@merge_request.to_reference}
+  You have been mentioned in Merge Request #{link_to(@merge_request.to_reference, project_merge_request_url(@merge_request.target_project, @merge_request))}
 
 = render template: 'notify/new_merge_request_email'

app/views/notify/resolved_all_discussions_email.html.haml

 %p
-  All discussions on Merge Request #{@merge_request.to_reference} were resolved by #{sanitize_name(@resolved_by.name)}
+  All discussions on Merge Request #{link_to(@merge_request.to_reference, project_merge_request_url(@merge_request.target_project, @merge_request))} were resolved by #{sanitize_name(@resolved_by.name)}

changelogs/unreleased/add-link-to-mrs-references.yml

+---
+title: Render Merge request reference as link in email templates
+merge_request:
+author:
+type: changed

spec/mailers/notify_spec.rb

@@ -467,6 +467,7 @@ describe Notify do
             is_expected.to have_body_text(status)
             is_expected.to have_body_text(current_user_sanitized)
             is_expected.to have_body_text(project_merge_request_path(project, merge_request))
+            is_expected.to have_body_text(merge_request.to_reference(full: false))
           end
         end
       end
@@ -534,6 +535,7 @@ describe Notify do
             is_expected.to have_referable_subject(merge_request, reply: true)
             is_expected.to have_body_text(project_merge_request_path(project, merge_request))
             is_expected.to have_body_text('due to conflict.')
+            is_expected.to have_body_text(merge_request.to_reference(full: false))
           end
         end
       end