Paginate the list of datasets

[LaurensRietveld]

There are two issues w.r.t. horizontal scalability (i.e. # datasets):

• Querying a non-existing dataset returns the complete (non-paginated) list of datasets
• Accessing the main page via html returns a non paginated list of all datasets

(both tested with ~ 650.000 datasets. You know which ;))

[RubenVerborgh]

Thanks for reporting, will look into this!