Make MergeUI/all vue components easier to test locally

[cdrini]

Work towards #9811

Technical

• Give MergeUI the same config url parameters that LibraryExplorer has for controlling data sources. This makes it easy to use the live-reloading dev environment, but have it hooked up to either prod or localhost to fetch data.
• Create a new script for testing vue components, npm run serve --component=MergeUI. No more needing to modify _dev.js!
  • Note: made use of an npm feature I learned about: If you call an npm script with an argument, like npm run serve --component=Foo, it automatically sets an environment variable to npm_config_component! Very handy. You can then access this from the script using the language's normal environment variable features, e.g. process.env.npm_config_component.
• Fix some long-standing bugs preventing vue live server usage on gitpod

Testing

• ✅ Tested the new script on git bash and on windows command line. Worked.
• ✅ Tested trying to do a merge failed because it hits localhost for the save call by default
• ✅ Tested LibraryExplorer works with the script.
• ✅ Tested the README instructions in gitpod.

Screenshot

Stakeholders

@benbdeitch @jimchamp

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 16.42%. Comparing base (ce16a79) to head (e725ba3).
Report is 385 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9854      +/-   ##
==========================================
+ Coverage   16.06%   16.42%   +0.36%     
==========================================
  Files          90       92       +2     
  Lines        4769     4907     +138     
  Branches      832      856      +24     
==========================================
+ Hits          766      806      +40     
- Misses       3480     3565      +85     
- Partials      523      536      +13     

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[benbdeitch]

I've repeated all the testing over Gitpod, and reviewed the code. Great job, it looks fantastic to me, and seems to work perfectly.