Replace ACLs and relationships with Entity Lists

[mrclay]

https://docs.google.com/document/d/1R3v_bYno6fw8mV5_GDW93uTI0pvR9S4yQ-BHMKTz1zs/edit#heading=h.yoq6inj5fhaf

This schema maps a target entity and string key to an ordered list of entities. This is similar to adding order to the current relationship table, and allows a single table to be used for relationships and more granular access control.

(I'm just dusting this idea off and giving it its own issue)