Close unclosed <td> tag

Close an unclosed <td> tag in the published story notification HTML
email template.

Addresses #580
Set image width using 'width' attribute

In the published story notification HTML email template, use the
``width`` attribute of the ``<img>`` tag instead of the ``max-width``
CSS property to be compatible with more email clients.  In particular,
certain versions of Outlook didn't handle constraining the widht with
``max-width``.

Addresses #580
Get rid of two-column layout in emails

In some mail clients, particularly the Android one, elements in the
two-column layout were falling outside, to the right of the display. To
avoid scrolling on mobile, move the content so everything is in vertical
succession rather than side-by-side.

Addresses #580
Add media queries to HTML email

This is to try to scale the content appropriately for mobile devices.

Addresses #580
Avoid circular dependency in management command

Move the import inside the method to avoid having issues with an
``ImportError`` being thrown when loading template tags. I only had this
problem on the dev server and not in my local development environment.

Addresses #580