Replace webpacker with esbuild

[jim]

What it does

This replaces the use of webpacker for bundling JavaScript and CSS with a combination of esbuild for JS and DartSass for CSS.

Why it is important

• The current webpacker config means that rebuilding the JS is pretty slow.
• Rails has deprecated webpacker and future energy is directed in other directions. This change aligns the project with upstream efforts.

Implementation notes

• We were previously using browserlist's defaults to translate new JS features into backward-compatible forms. With esbuild, the options are more limited. Using --target=chrome58,firefox57,safari11 to restrict what JS features are used should be OK for now and is more conservative than what we were shipping before.

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