Fix: unexpected escaped HTML tag

Add `html_safe` method to allow sending santized HTML tag.

app/views/notify/issue_due_email.html.haml

 %p.details
-  = sprintf(s_("Notify|%{author_link}'s issue %{issue_reference_link} is due soon."), { author_link: link_to(@issue.author_name, user_url(@issue.author)), issue_reference_link: issue_reference_link(@issue) })
+  = sprintf(s_("Notify|%{author_link}'s issue %{issue_reference_link} is due soon."), { author_link: link_to(@issue.author_name, user_url(@issue.author)), issue_reference_link: issue_reference_link(@issue) }).html_safe
 - if @issue.assignees.any?
   %p