Add options for filtering variants

[nawatts]

Allow filtering gnomAD variants list based on additional columns, such as clinical significance (#581) and flags (#400). Numeric columns, such as AC, should allow filtering to less than/equal to/greater than a value.

[nawatts]

Also related, once filtering on each column is available, the search box can become search instead of filter, with options to jump to next/previous matches (#312).