(#61) Better templates for "verify your account" emails:
- Send a multipart text/html email (instead text only)
- Added some styling to the html version of the email,
based on our current login page styles.
Important: as the html version contains html code generated using
request.resource_url() and request.static_url(), when running in
development mode under http://localhost, the links to images
and css files in the rendered email point to localhost, so they
won't work outside the machine running openworkouts.