Improve the access-limiting implementation

[chrisroos]

We currently use a single access_limited Array to determine whether an asset is access-limited, where:

• An empty array means that it's not access limited.
• An array containing 1 or more user UIDs means that the asset is only visible to the users represented by the UIDs in the array.

This is very similar to the way access-limiting works in the Content Store, with one difference being the lack of auth_bypass_ids in Asset Manager as mentioned in #471.

I think the current implementation is slightly confusing and think it might be better to model it using two fields: one to control whether the asset is access-limited and the other to contain the UIDs of authorised users.