Revert to using CleanWebpackPlugin

[victorlin]

Description of proposed changes

This reverts "Replace CleanWebpackPlugin with output.clean" (798f3ee) due to issues with Hot Module Replacement and output.clean.

Related issue(s)

• Closes [dev] hot-reloading broken #1737

Checklist

• hot reloading works (can no longer repro [dev] hot-reloading broken #1737)
• Checks pass
• If making user-facing changes, add a message in CHANGELOG.md summarizing the changes in this PR no functional changes
• (to be done by a Nextstrain team member) Create preview PRs on downstream repositories. not necessary

[jameshadfield]

Thanks @victorlin!

One of the reasons I've been hesitant to dive back into webpack is due to bugs such as this. Hot reloading, extendability, aliases, transpilation, env-variable handling, bundling and differential cacheing all rely on webpack and the interaction between webpack and a dozen other packages. From experience it's all too easy to break some of this functionality inadvertantly. This part of the JS ecosystem keeps evolving, and so we fall out of date over time. (Notice all the deprecation warnings we are getting both at build time and run time.)

I wonder if the best way to update this part of Auspice is a concerted push to update all of React, hot reloading, redux (?), babel, webpack, webpack plugins, etc. As part of this we'd define a number of tests beforehand so we can be more confident nothing's broken.

[victorlin]

A concentrated push sounds like the right approach here (instead of incremental pushes such as #1733). That would require much more effort and intention. I've added it to the "Project ideas" section in the priorities doc:

• Modernize front-end codebases
  • TypeScript adoption in Auspice
  • Replace outdated usage of React, Webpack, Babel, Redux, etc.