Make JavaScript build pipeline less confusing

[zarino]

There are files in /vendor/*/js/ that aren’t used any more, because we compile our JavaScript with script/build_js and that uses esbuild to include third-party libraries from node_modules (I think?).

Meanwhile, code seems to be scattered at random throughout area.esm.js, explore.esm.js, and home.js (note, no esm?) and pages like area.html and explore.html load both home-out-esm.js and their page-specific file.

JavaScript and CSS files seem to be handled differently between local development (with DEBUG=True), local development (with DEBUG=False) and the live environment.

It’s also awkward (during local development) that the script/build_js step is manual, and separate from the Django staticfiles/compressor pipeline. See #486.

Ultimately, this is all just a bit confusing, and not documented in the README or in comments anywhere, and I just wasted a few hours today trying (and failing) to diagnose a weird JavaScript behaviour, and this opaqueness didn’t help.