Add ability to get stitched together CDS, protein, and cDNA sequences in feature details

[cmdcolin]

Requested by #1755

Example screenshot:

There are more things that could be done including getting promoter/intron sequences (add +/- some user-configurable number of bases) around the region, but I think this is possibly a good start

[codecov]

Codecov Report

Merging #1758 (7966a19) into master (a1ae57d) will decrease coverage by 0.19%.
The diff coverage is 30.21%.

@@            Coverage Diff             @@
##           master    #1758      +/-   ##
==========================================
- Coverage   58.74%   58.54%   -0.20%     
==========================================
  Files         455      455              
  Lines       21049    21140      +91     
  Branches     4987     5011      +24     
==========================================
+ Hits        12365    12377      +12     
- Misses       8375     8453      +78     
- Partials      309      310       +1     

Impacted Files	Coverage Δ
...lignmentsFeatureDetail/AlignmentsFeatureDetail.tsx	41.79% <ø> (ø)
...archicalTrackSelectorWidget/components/Category.js	86.66% <ø> (ø)
...src/LinearGenomeView/components/SequenceDialog.tsx	48.71% <ø> (ø)
...quenceRenderer/components/DivSequenceRendering.tsx	93.25% <ø> (-1.34%)	⬇️
.../src/VariantFeatureWidget/VariantFeatureWidget.tsx	45.20% <ø> (ø)
products/jbrowse-web/src/jbrowseModel.js	65.85% <ø> (ø)
...kages/core/BaseFeatureWidget/BaseFeatureDetail.tsx	44.97% <18.01%> (-24.38%)	⬇️
packages/core/util/index.ts	81.81% <78.57%> (-0.61%)	⬇️
... and 2 more

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