1381 improve assembly add form

[elliothershberg]

This PR improves the form and workflow for adding a new assembly graphically.

Previously

Would accept a name, and then immediately direct the user to the full configEditor for the assembly.

Now

A simple form that asks for name and adapter info (IndexedFastaAdapter, BgzipFastaAdapter, or TwoBitAdapter)

when submitted, the user gets notified:

and then returned to the main table of assemblies. Any more granular editing can be accessed through the pencil icon which gives access to the full configEditor.

[codecov]

Codecov Report

Merging #1439 (8ab6044) into master (9b9818b) will decrease coverage by 0.06%.
The diff coverage is 59.09%.

@@            Coverage Diff             @@
##           master    #1439      +/-   ##
==========================================
- Coverage   59.58%   59.52%   -0.07%     
==========================================
  Files         425      425              
  Lines       18990    19029      +39     
  Branches     4442     4454      +12     
==========================================
+ Hits        11316    11327      +11     
- Misses       7384     7409      +25     
- Partials      290      293       +3     

Impacted Files	Coverage Δ
...management/src/AssemblyManager/AssemblyManager.tsx	73.68% <ø> (-18.43%)	⬇️
products/jbrowse-desktop/src/jbrowseModel.js	19.29% <0.00%> (-0.35%)	⬇️
...management/src/AssemblyManager/AssemblyAddForm.tsx	66.66% <60.00%> (-33.34%)	⬇️
products/jbrowse-web/src/jbrowseModel.js	73.61% <100.00%> (+0.37%)	⬆️
...-management/src/AssemblyManager/AssemblyEditor.tsx	33.33% <0.00%> (-66.67%)	⬇️
plugins/alignments/src/BamAdapter/BamAdapter.ts	69.51% <0.00%> (-1.22%)	⬇️
...ments/src/SNPCoverageAdapter/SNPCoverageAdapter.ts	89.92% <0.00%> (-0.78%)	⬇️
packages/core/util/index.ts	83.95% <0.00%> (ø)

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