Improve user mailing experience

These will be used for both MailChimp and Mandrill/ActiveRecord mails. 

The aim here is to have a master "template generator" that will basically spit out HTML emails with all the messy code. Accordingly, I've used PHP includes as a simple way of generating the static pages—this way changes will impact all the templates as a whole. 

Will be tested using Litmus/Inbox Inspector prior to implementation, and all mails are responsive and as unreliant on images as possible... though this may change at a later date.

Solves #76 and #93 when it's all done.