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.
I think it'll be okay because HTML won't care about the whitespace. I'll leave this as-is but watch out for this.