#pontoon bug 1216546 - progressive load of entities

gistfile1.txt

Backend:
* pagination has to be done on real queryset instead of a serialized array 
* all filters should be reflected on the backend side.
* has_suggestions and whole serialization of entitieos should be done as the last step of the entities view.
Frontend:
* We should take an mixed approach - load only entities when it's needed
* We should keep an state of currently loaded page
* Load entities only if there are entities to load1
* Search/filtering should reset a page counter.

Current todo:
* gather-up some plan.