Collections API/model changes

[dfernandezm]

Given I have a collection with items
When I mark one as swappable
Then I can see swappable items across my collections

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This way we can identify repeated items easily.

Given I have a collection
when I add an item to it
if that item already exists in this or other collection
quantity should be increased by one

Collection model

• Add categories/taxonomy like Conmemorative Coins of Portugal. In my Collection of Coins, I can create a Category for Coins of Canada, Quarters of USA, etc.
• Tags to search for items
• Extra attributes like Location (of an item), physical/geographical -- free text
• Concrete: BanknoteItem / CoinItem