Create paged region API

[pjvandehaar]

1. paging: URL /api/region/<phenotype>/chr1-23432-end?max_num_variants=1000 returns {data: [...], next:'/api/region/<phenotype>/chr1-98432-end?max_num_variants=1000'}.

2. Make a file (and API endpoint) for each phenotype only including pvals < 1e-6.

3. Make a single file (and API endpoint) for all phenotypes at once only including pvals < 1e-6. (1e-6 is plenty, because: 0.05 / 1000 traits / 1000 variants = 5e-8)