Overhaul Webpack config; configure & apply linters

[brlodi]

This represents essentially a ground-up rewrite of our Webpack configuration to address a number of issues that were reducing or eliminating the benefits of building with Webpack and unnecessarily bloating the output bundles.

Specific points of interest:

• Webpack config now exports functions instead of objects to allow for the use of environment variables within the config (the standard Node exposure of ENV is not available at the time the Webpack config is loaded, but when provided as a function the function is called with the command line environment)
• Webpack now configured to split bundles as necessary to minimize both initial load times and the amount of duplicated code across different logical bundles (e.g. common libraries like JQuery are now split out into their own bundle automatically so they only get downloaded once no matter how many entry points are on the page)
• Configured Babel appropriately to minimize code repetition across output files
• Updated Browserslist with a more practical set of targets, and updated various packages that supposedly read the .browserslistrc file to the versions that actually do it
• Included ESLint to our build pipeline. The packages were there, but nothing ever ran ESLint unless you manually ran it from the CLI
• Updated ESLint config with a proper, fairly comprehensive styleguide
• Added StyleLint to enforce code style of our SCSS files; included in build pipeline
• Reduced somewhat the log output during development builds, since many of the sections were uselessly turncated anyway.
• Quieted the log output during development builds, and made them fail on almost all errors to help ensure we don't ship broken builds
• Added "staging" and "production" flags to the NPM scripts, which are available in the Webpack configuration to modify behavior as necessary.
  • Implemented S3 URL injection for Webpack assets so our CSS doesn't always point to the dev bucket even in production (allows a solution for

    387, but this doesn't add any logic to the build/deployment scripts to

    actually generate a staging build)

• Reimplemented our use of FontAwesome to enable Webpack to tree-shake it, reducing its footprint in our output bundle from almost 1MB to a few kilobytes
• Explicitly excluded Moment.js locales from our builds in a way that they can still be explicitly imported if needed but otherwise won't be included in output bundles, reducing the Moment.js footprint in our bundle by ~70%
• Trimmed down some dependencies that we weren't using or could trivially stop using, and added a couple to support compiling single-file Vue components
• Configured Autoprefixer (and banned manual prefixes in the linter) to allow us to use only standard syntax in authored SCSS and have necessary vendor prefixes handled automatically for our target browsers in .browserslistrc
• Somewhat fixed minification of both JavaScript and CSS
• Added WebpackBundleAnalyzer to enable visualization of the contributing sizes of modules in our bundle. Enabled by default in development, but can be easily turned on in production and staging builds by a command line flag
• Assorted changes to templates and SCSS files to support the new Webpack settings and to conform to linting rules

[vforgione]

If your PR comment all holds then I declare you to be the Front End Messiah.

[brlodi]

Glamour shot of tree-shaking actually working:

(That's just under 1MB all told, except the atrociously huge GIFs we're using for error messages)