Add an option to add a URL for the item thumbnail

[CabtonWithaK]

CONTEXT
This feature is meant to improve the management of inventory items. Specifically for TPOS, where thumbnail URL's are already used on items that aren't managed with this inventory extension.

AS IS
For an item I can only upload a (series of) thumbnail image(s)

TO BE
For an item i can choose to set a URL for the thumbnail.

For example add a Toggle: URL | UPLOAD

Provide URL textfield when URL option is set.