Collection image storage

[jmankoff]

Collections need some sort of information about what image represents them. Ideally the following should be true:

• There is a default image assigned.
• If the default has no alt text, each time a new product or products are added, we check for whether any of them have alt text and swap the first one we find for the default
• The front end can also upload an image or provide an image url for a collection images (with alt text). In this case, no further default swapping happens.