Create --branch and --nightly flags for jbrowse create and jbrowse upgrade commands

[cmdcolin]

This re-instates the upload of a jbrowse-web build to the folder where it is being deployed (this existed in travis but was lost in the github move)

Then it also adds a --branch and a --nightly (alias for --branch master) flags to the jbrowse create and jbrowse upgrade commands

[codecov]

Codecov Report

Merging #1579 (4398493) into master (25d67d7) will decrease coverage by 0.00%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master    #1579      +/-   ##
==========================================
- Coverage   59.44%   59.44%   -0.01%     
==========================================
  Files         435      435              
  Lines       19620    19628       +8     
  Branches     4587     4593       +6     
==========================================
+ Hits        11663    11667       +4     
- Misses       7663     7667       +4     
  Partials      294      294              

Impacted Files	Coverage Δ
products/jbrowse-cli/src/base.ts	84.72% <0.00%> (-2.43%)	⬇️
products/jbrowse-cli/src/commands/create.ts	88.88% <100.00%> (+0.79%)	⬆️
products/jbrowse-cli/src/commands/upgrade.ts	82.35% <100.00%> (+1.70%)	⬆️
...inearGenomeView/components/RefNameAutocomplete.tsx	86.20% <0.00%> (-3.45%)	⬇️

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[rbuels]

This is great functionality, I think just tweak the docs to make it clear that using these flags get a "development build" or an "experimental build" that isn't official