Move html_safe declaration for user_mailer from layout to helper

This allows us to enable output safety checks for all code within
erb files.

`rubocop --auto-gen-config` ignores code within the erb files, so
it is easier to maintain an exclusion on the helper than inside
the layout.

app/helpers/user_mailer_helper.rb

@@ -29,7 +29,7 @@ module UserMailerHelper
     # Because we can't use stylesheets in HTML emails, we need to inline the
     # styles. Rather than copy-paste the same string of CSS into every message,
     # we apply it once here, after the message has been composed.
-    html.gsub("<p>", '<p style="color: black; margin: 0.75em 0; font-family: \'Helvetica Neue\', Arial, Sans-Serif">')
+    html.gsub("<p>", '<p style="color: black; margin: 0.75em 0; font-family: \'Helvetica Neue\', Arial, Sans-Serif">').html_safe
   end
 
   def style_left