Fix loading of local files in jbrowse-desktop

[cmdcolin]

fixes #1315
fixes #1344

[codecov]

Codecov Report

Merging #1505 (52253c9) into master (ffbe57d) will decrease coverage by 0.01%.
The diff coverage is 23.52%.

@@            Coverage Diff             @@
##           master    #1505      +/-   ##
==========================================
- Coverage   59.47%   59.46%   -0.02%     
==========================================
  Files         435      435              
  Lines       19547    19550       +3     
  Branches     4581     4583       +2     
==========================================
- Hits        11626    11625       -1     
- Misses       7622     7626       +4     
  Partials      299      299              

Impacted Files	Coverage Δ
packages/core/ui/FileSelector.tsx	42.30% <ø> (ø)
packages/core/util/io/ElectronLocalFile.ts	3.70% <0.00%> (-0.15%)	⬇️
...management/src/AssemblyManager/AssemblyManager.tsx	73.68% <ø> (ø)
...lot-view/src/DotplotView/components/ImportForm.tsx	12.82% <0.00%> (-0.34%)	⬇️
...ew/src/LinearSyntenyView/components/ImportForm.tsx	6.81% <0.00%> (-0.16%)	⬇️
...management/src/AssemblyManager/AssemblyAddForm.tsx	66.66% <50.00%> (ø)
...ins/wiggle/src/LinearWiggleDisplay/models/model.ts	81.98% <0.00%> (-0.91%)	⬇️

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[cmdcolin]

Went ahead and fixed the ts-ignores with a couple helpers