Don't throw on UCSC connection containing assemblies not matching connection->assemblyNames config

[cmdcolin]

Also fixes session->connections getter in jbrowse-web to return sessionConnections and some other misc refactoring

Fixes #3737

[codecov]

Codecov Report

Merging #3738 (924e0e9) into main (b8174a5) will decrease coverage by 0.05%.
The diff coverage is 52.50%.

@@            Coverage Diff             @@
##             main    #3738      +/-   ##
==========================================
- Coverage   64.38%   64.33%   -0.05%     
==========================================
  Files         977      977              
  Lines       29892    29897       +5     
  Branches     7166     7172       +6     
==========================================
- Hits        19246    19235      -11     
- Misses      10480    10496      +16     
  Partials      166      166              

Impacted Files	Coverage Δ
...get/components/dialogs/ToggleConnectionsDialog.tsx	0.00% <0.00%> (ø)
...s/linear-genome-view/src/LinearGenomeView/model.ts	84.40% <ø> (ø)
packages/product-core/src/Session/Connections.ts	32.65% <14.28%> (-7.35%)	⬇️
plugins/data-management/src/ucsc-trackhub/model.ts	75.47% <77.77%> (-1.28%)	⬇️
packages/web-core/src/BaseWebSession/index.ts	73.04% <100.00%> (+0.47%)	⬆️
...ns/legacy-jbrowse/src/JBrowse1Connection/model.tsx	75.00% <100.00%> (ø)
products/jbrowse-cli/src/base.ts	84.41% <100.00%> (ø)
...roducts/jbrowse-cli/src/commands/add-connection.ts	89.04% <100.00%> (-4.94%)	⬇️

... and 1 file with indirect coverage changes

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[cmdcolin]

note that technically, we could load all tracks from connection even if the 'assembly wasn't found' ...then in the rare case that the assembly is loaded later via assembly manager, it would automatically have the new tracks, but may be ok for now