app/views/review_mailer/review_email.html.erb
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
</head>
<body>
<p>Your code for <%= @review.submission.exercise.name %> has been reviewed.</p>
<p>See <%= link_to @url, @url %>.</p>
</body>
</html>