Show previews of banners in generator and links on dashboard.

Image banners use a 125x125 variation as their preview, as do upgrade banners. Text banners show the text in a small box. All banners attempt to show a localized preview and fallback to en-US.

The tests use an in-development version of `factory_boy` for a lovely little decorator that mutes signals. Otherwise, the ImageField on variations would be attempting to read files that don't exist in order to populate the width/height fields when models are saved.