Do not load all elements in a dataset

Refs https://redmine.teklia.com/issues/5686
Depends backend#1677 (closed)

Large datasets are super slow to load on the frontend, as ALL pages need to be loaded to display the real counts and the elements. In most cases, users are only interested in viewing a few elements and getting the total number of elements.

A patch should:

1. use the new stats provided by RetrieveDataset
2. Only load the 3 first pages of elements, then display a button to "load more" at the bottom of page which load 1 more page at a time