Add ability to find all variants that co-occur with a variant

[nawatts]

Given a variant, find all other variants that should have co-occurrence results with that variant (can't simply search the co-occurrence index). Show a table of those variants with links to the co-occurrence variant pair page.

This should be a separate page. It can be linked to from the variant page (replace the current link to the variant pair page).

[nawatts]

Will require data pipeline changes to index co-occurrence records by genes in which both variants appear.