switch to Webpack v5

[derhuerst]

This PR includes the changes of and depends on PR #4777; Please review & merge that one first.

Proposed Changes

• upgrade to Webpack & Webpack CLI v5, adapt config according to the migration guide (some things left to do)
• upgrade to webpack-dev-server v4
• upgrade plugins: {favicons,optimize-css-assets,terser}-webpack-plugin, webpack-assets-manifest, webpack-bundle-analyzer
• convert all files (transitively) imported by webpack.config.babel.js (and thus by app/configurations/config.*.js) to pure ESM, instead of an ESM/CJS mix as before (still some work to do)

Pull Request Check List

• A reasonable set of unit tests is included
• Console does not show new warnings/errors – didn't thoroughly check yet
• Changes are documented or they are self explanatory
• Code coverage does not decrease (unless measured incorrectly)

Review

• Read and verify the code changes
• Test the functionality by running the UI locally with all popular browsers available in your platform
• Check that the implementation matches the design, when such one is defined in a Jira issue
• Merge the pull request

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(This is the upstream equivalent of embarkokc#24.)