update: don't serialize permissions on resources within container

Closes #299 (closed)

We think that Serializing permissions on every resource in a container is expensive. This MR changes the behaviour of LDPSerializer so that permissions are not serialized on child resources if they're in a container (/circles/, /circles/1/members/), but they are serialized if the child is called directly (/circles/1/)

@jbpasquier before we test this with the front-end, is there a staging that's already deployed with the performance test data (500 users, 500 projects), or if not is there one that I can use to measure the improvement please?