This avoids having two `application.js` files (one in `webpack/` and one
in `app/assets`, which is confusing/error-prone) in favor of using
unique/unambiguous filenames.

This will be used to produce an asset manifest mapping entry point names
to hashed filenames, for use with the external_asset_pipeline gem. (The
current asset manifest produced by the StatsPlugin is in a different
format that includes a bunch of extra information which isn't needed by
and isn't compatible with external_asset_pipeline.)

This will be used to replace webpack-rails, which is no longer
maintained.

instead of webpack-rails (as the latter is no longer maintained, so
we're replacing it with the former).

These were outdated after we switched from webpack-rails to the
external_asset_pipeline gem.

This was only needed for webpack-rails, which we've stopped using in
favor of the external_asset_pipeline gem (which uses the manifest
produced by the webpack-assets-manifest plugin).

Both webpack-assets-manifest and external_asset_pipeline assume a
default manifest filename of `manifest.json`. The only reason we weren't
using this default before was because we had been using the StatsPlugin
to produce a different `manifest.json` file. Now that we've removed the
StatsPlugin, we can rely on the default webpack-assets-manifest and
external_asset_pipeline configuration.

We stopped using webpack-rails, but we still want these entries in the
gitignore, so we just made the comment more general.

We've replaced this gem (which is no longer maintained) with the
external_asset_pipeline gem