Cache remote record instances

[boardfish]

Some sort of serialization so that multiple instances that reference a single remote resource use the same data.

[boardfish]

@johndbritton We talked about this issue again earlier, but I've since been thinking. How can we prevent users from accessing data that they shouldn't? Say we have a remote resource that we fetch based on the user's token - caching that data to the database will mean that if that record is shared among several users by way of has_many :through, the cached data will then be accessible to all those users, even if authorization is user-dependent.

[boardfish]

Unrelated to the above, but I should add here that a couple of things we talked about with this included:

• fetching methods (see the Offline Cookbook for inspiration. There are some neat async avenues to explore here.)
• caching the data (JSONB seems to be the way to go, at least for us, but we should consider how others might do it)