[bug 1044997] Fix weekly digest emails

This does a bunch of things:
- Prevents the cron from sending an email if `settings.STAGE == True`
- Fixes the URLs in the emails (both HTML and plain-text)
- Fixes the formatting of the plain-text email to match the HTML email

r?
I was going to nit at you about indentation, but then I realized this is a text email. Then I was :frowning:.