Mailer updates

:fork_and_knife: 

Minor updates to improve the code and display of emails.

- ensure `@to` is set in mailers to set the email in the footer (it was missing in a couple of places)
- link to Supermarket in the footer of emails
- link to Supermarket from the logo in the HTML header
- added `()` to `mail()` where they were missing. Consistency :cool: