Add Closure Compiler to our pipeline

[fzzzy]

(edit: This bug was originally about prepack, but it doesn't work and Closure Compiler does, so this bug is now about that.)

https://prepack.io

[fzzzy]

Preact is too alpha. It has an exception when I try to run it on a mashed-together app.js and vendor.js bundle.

I also tried closure compiler, and it also has an error. I may be able to fix it by upgrading react. Testing...

[fzzzy]

Upgrading react fixed the problems with using closure compiler on the bundle. It cuts the app+vendor bundle size in half, from 2.7 meg to 1.3 meg.

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Here's the command line I am using, for future reference:

java -jar ../compiler-latest/closure-compiler-v20170806.jar --js frontend/build/static/app/app.js --js_output_file frontend/build/static/app/app-cc.js

[fzzzy]

Les was able to switch to webpack, which also does tree-shaking, so we don't need this any more. The webpack branch shrinks our bundle sizes.