While it can be useful to add immutable entities (such as documents) to entity sets, entity sets have never really supported that. The entire UI side of entity sets assumes that entity set items can be mutated by users. This led to inconsistencies and bugs. For example, adding a file to an entity set would remove the `contentHash` property, and once added to an entity set, users were able to change properties of files such as the file name.

When expanding an entity in a network diagram, we need to be able to include only non-file entities. This change adds a `schemata` query parameter to the entities expand API endpoint, the endpoint used by the network diagram UI, to enable this.

This change is a little hacky and adds even more complexity to the `expand_proxies` method which already is quite complex. I'm not entirely sure this is a good idea.