Feature/markdown in mention emails

this is a bit of a bulky commit. it falls into several parts:

- adding the Gem Roadie
- modifying emails
- automating the sending of suite of mock emails

#### Roadie

easy, essentially allows us to have an email.css file which works in html emails 

#### modifying emails

minor tweak to the text unsubscribe footer. a lot of tweaks to the html emails to take advantage of css, copying defaults from existing Loomio css. some minor layout fixes done which put tables into emails to make them clearer to read.
worked on @rdbartlett with this. if anyone wants to see the suite of emails I can use the following part to send them to you

#### automating mock emails

after a lock of mucking about settled on stub_model/ mock_model and stubs. The file is run in test ENV locally and you specify whether sendgrid or mailcatcher is desired send method. see the top of the file for commands as to how to kick it off.

ideally need to find a way to send just particular emails, but this is a project for later. (firing up the rails test env is actually the prohibitive step atm)

modify list of addresses in `email_test.rb` file to send to more/ less emails