Set priority of mutation types in genes

[wir963]

Each mutation type should be associated with a number, which indicates its priority or seriousness. ExploSig should always show the top priority mutation for a given sample. We should also use this priority when we sort by mutation type (screenshot https://cl.ly/0309d52f8964). This should resolve part 1 of issue #86.

I propose we use the following priority (in rough order of impact)
Frameshift > Nonsense > Nonstop > in-frame indel > Missense > Splice Site > Translation Start Site > Other mutation > Silent > None

[keller-mark]

Done in lrgr/explosig-server@9aee803#diff-264f618d47c09314e7e4d290f7a64ab6