Switch to using the Wagtail Image for thumbnails

[zerolab]

Using WagtailImageField instead of a plain FileField would allow us to tap into the Wagtail renditions model and benefit from proper resizing, better thumbnail display across the admin

Would need:

• a data migration or management command
• decision on what to do with the uploaded thumnbails and the Wagtail Image listing -- is it an issue to have all of these thumbnail images show up?