Remove unused AbstractProductImage properties for thumbnailing

Images are resized on the template level with Sorl, so having
those properties only leads to confusion. They were not used by stock
Oscar. Furthermore, thumbnail_url returned the full-size image URL,
which is wrong.

Fixes #1011
Quantize after calculating tax

Previously, returned prices would suddenly have more decimal places.
Quantizing appropriately fixes this.

Fixes #994
Ensure migrations reference user model wrapper
Merge remote-tracking branch 'upstream/master'