Use `to_reference` for Issue "show" and "edit" page titles

Thanks to Greg Laubenstein for noticing in !6503

app/views/projects/issues/edit.html.haml

-- page_title "Edit", "#{@issue.title} (##{@issue.iid})", "Issues"
+- page_title "Edit", "#{@issue.title} (#{@issue.to_reference})", "Issues"
 
 %h3.page-title
   Edit Issue ##{@issue.iid}

app/views/projects/issues/show.html.haml

-- page_title           "#{@issue.title} (##{@issue.iid})", "Issues"
+- page_title           "#{@issue.title} (#{@issue.to_reference})", "Issues"
 - page_description     @issue.description
 - page_card_attributes @issue.card_attributes