RP branded emails.

* Create a new model on the front end, rp_info.js, to keep things sane.
* Add new backend validation types: color and image
* Add backend tests to check backgroundColor and siteLogo
* Update the copy in HTML and text emails as per @ryanfeeley's suggestions.
* Loads of email styling updates to work across webmail and mobile.
* Use a common _header.html.ejs and _footer.html.ejs shared across all emails so updates are simpler
* Simplify the email template configuration - only specify the template filename, not the full path.