Improve support for large region requests

[nawatts]

Currently, the region page will only load a maximum of 5000 variants, however large a region is requested. This is done to prevent someone from, for example, requesting a region spanning all of chromosome 1 and its 1,687,282 variants (for comparison, TTN has 52,780 variants).

https://github.com/macarthur-lab/gnomadjs/blob/cd016641c10ffe41f07af9f75f9858372ade6922/packages/api/src/schema/types/region.js#L214-L221

https://github.com/macarthur-lab/gnomadjs/blob/cd016641c10ffe41f07af9f75f9858372ade6922/packages/api/src/schema/types/elasticVariant.js#L591-L594

Furthermore, for some region sizes it will only load variants of certain consequences.

https://github.com/macarthur-lab/gnomadjs/blob/cd016641c10ffe41f07af9f75f9858372ade6922/packages/api/src/schema/types/elasticVariant.js#L578-L588

However, nowhere in the browser is it indicated that either the number of variants or the consequence is limited. Thus, there's no explanation for why the individual variant track does not match up with the binned variant track or the total number of variants shown at the top of the page.

Options:

• Show in the browser that a maximum of n variants are loaded on the region page (we could set this limit higher than 5k given that we display 50k on TTN's gene page)
• Limit region requests to a size where all variants can be loaded (the current gnomAD browser blocks requests for > 100kbp)
• Do not load individual variants and remove the variant table for regions greater than some threshold (still show coverage, genes, and binned variants tracks)
• Load a page of variants at a time instead of all variants up front (this is probably where we want to go long-term)

[nawatts]

This now returns either all of the requested variants or no variants and an error message if there are too many.

If there are too many variants to show individual variants, a binned variant track should be shown.