[WIP] Unify Julia + NPM publishing.

[twavv]

We shouldn't have to check built-in files to Git.

The easiest thing to do is to publish things to NPM and then download the current version when being installed.

[twavv]

I think publishing from CI is not gonna be the best route, here.

Instead I'm working on a publish.jl script which takes care of building the JS, testing against it, setting the NPM package versions, pushing to NPM, and creating the tag.

The reason why doing it in CI is problematic is because of the assumptions that Lerna make (which make sense when Lerna is the only tool being used to version... but... alas...).

[codecov]

Codecov Report

Merging #319 into master will increase coverage by 0.02%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master     #319      +/-   ##
==========================================
+ Coverage    62.6%   62.62%   +0.02%     
==========================================
  Files          17       16       -1     
  Lines         599      594       -5     
==========================================
- Hits          375      372       -3     
+ Misses        224      222       -2

Impacted Files	Coverage Δ
src/iframe.jl	100% <ø> (ø)	⬆️
src/providers/generic_http.jl	73.07% <ø> (ø)	⬆️
src/WebIO.jl	77.77% <ø> (ø)	⬆️
src/providers/mux.jl	11.76% <0%> (+0.65%)	⬆️
src/devsetup.jl	50% <100%> (-10%)	⬇️

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[twavv]

Closes #320.
Closes #317.
Closes #315.
Closes #314.
Closes #313.

[twavv]

Closes #264 (we always build before testing if checked out for development).

[twavv]

The VM which runs Julia 0.7 on Mac is apparently very flakey (it's happened twice where we get errors when it tries to download stuff from Conda). Going to merge this as is.