Convert prompt and welcome emails to html

Because:

- We'd like to make some style changes to our outgoing emails that
  require html.

- It seems as though SendGrid is converting our outgoing messages to
  anyway.

This commit:

- Converts the prompt and welcome emails to html.

Style changes to come in a future PR.
It may not matter with HTML emails, but should we remove the indent on this? Email clients seem to find the worst possible way to render text every time.