Restyled mail templates

Restyle of all mail templates.

* Used simple fluid design (looks good on small and wide screens).
* Moved all style to one place (src/Frontend/Core/Layout/Templates/Mails/Header.tpl).
* Removed all inline styles.
* Prints site name+domain in footer.

Before/after compare, see this PDF file:
https://github.com/bart-lysander/files/blob/master/restyled-mail-templates.pdf?raw=true