add missing columns in mutation mapper table

[leexgh]

adding exon, hgvd, gnomad, clinvar, dbsnp to mutation mapper table

fix: cBioPortal/cbioportal#6868

[inodb]

Thanks @leexgh ! This solves the issue temporarily, but I think the actual issue is that we keep forgetting to add the shared columns in all three locations: Patient View, Mutations Tab and Mutation Mapper. To avoid that from happening again it might make sense to either:

1. Introduce a variable that's called something like SHARED_MUTATION_COLUMNS and add that in all three places or
2. Use inheritance and change the base class in some way to always include them