add pagination to entries JSON response

[Pomax]

We're currently missing a {page,per_page} pair for the /entries route, which means there is no way to only grab 10 (or 5 or 20) entries at a time. We'll need to add something like https://docs.djangoproject.com/en/1.10/topics/pagination/#using-paginator-in-a-view to make sure we only send managable sets to the client.

[xmatthewx]

related front-end things mozilla/network-pulse#235

cc @mmmavis