add support for organization members

[maxlaverse]

Add support for organization members, in order to source them by email, and re-use them in organization collections. This removes the on-the-fly lookups of members on organization collections and brings use closer to how resources are modeled. It simplifies the code, and will allow us to use org_members in other places.

Also:

• Fix items not being updatable when one of their collection ID is changed
• Make ordering irrelevant when specifying members of a collection
• Fix some Locks right when doing ACL-related operations