mail: allow sending a text version of mail

[jonathaningram]

Extracted from #1280

Cleaned up the mailer tests.

[jcscottiii]

@jonathaningram this looks good!
i have a question: we are thinking about moving completely to text only e-mails (thanks for this PR for moving it to send text e-mails!) and ripping out the HTML template. now if you still want the html template, we can keep it. Just want to plan accordingly.

[mogul]

I think we want to keep HTML mails, just make the default no frills!

[jonathaningram]

@jcscottiii yep I think we should keep the ability to send HTML emails too.

On a semi-related note (because you mentioned that you might be making a change to sending text-only emails), you might like to take a look at these links for a PR that I have yet to make, but probably will at some stage:

• d919da5
• https://github.com/govau/ci-console/blob/master/skin/email-templates/src/emails/UserAssociatedWithOrganization.js
• https://github.com/govau/emailtemplate

The first link shows how I've incorporated a new emailtemplate package into the dashboard so that skins can provide the HTML, text and subject as separate files (basically, how to get the hard-coded "cloud.gov" subject out of Go code).

The second link shows how we are actually building our HTML email templates - in short, we use React to write out a HTML file, and the node stack also writes out the subject and text files.

The third link shows the actual Go package. It has documentation/examples so hopefully it's clear once you click through.

Let me know what you think!