Implement proper paging for entities

[mathjazz]

We currently use a weird way of paginating entities - in each request we include a list of already loaded entities, which we exclude from the matching entities in Django and then return the first page of the remaining entities.

This is a relic of the react-infinite-scroller library which we no longer use.

We should implement proper paging, i.e. drop exclude_entities and only return the relevant page.