Make work with unified gnomad/exac names

[holtgrewe]

Is your feature request related to a problem? Please describe.
In varfish-org/varfish-server-worker#307, we unify the strucvar background database names of gnomad and exac. This must be reflected in the varfish-server as well.

Describe the solution you'd like
Adjust the query parameters and forms.

• gnomad => gnomad_genomes
• exac => gnomad_exomes

Describe alternatives you've considered
N/A

Additional context

• Unify names in strucvar filtration background database exac/gnomad varfish-server-worker#307
• When rolling out, all previous strucvar queries should be cleared. Ensure that rollback is possible.