Use Spring Mail Sender instead of Java Mail

Should simplify configuration of the smtp server
Move Thymeleaf templates in preparation for adding email templates.

https://www.pivotaltracker.com/story/show/69799980
Use Thymeleaf templates for sending email

Specify the sender as admin@(host of login.url from login.yml)
Merge branch 'feature/thymeleaf_mail' into develop