Add support for contenthash for b2share-bundle.js building

[hjhsalo]

1. Use webpack 'contenthash' -variable to add id-number to b2share-bundle.js and b2share-bundle.js.map files during webpack build.
2. Use webpack to inject b2share-bundle-<CONTENTHASH>.js dynamically to index.html (

   b2share/webui/app/index.html

   Line 97 in d846068

   <script src="/b2share-bundle.js"></script>

   )

html-webpack-plugin -webpack module could be used for 2.
It supports lodash templating out-of-the-box.

v.3.2.0 seems to be the last compatible version with webpack v1 which we still use: https://github.com/jantimon/html-webpack-plugin/blob/master/CHANGELOG.md#400-2020-03-23 --> "Dropped support for Webpack 1 - 3".

See this and this for more info on templating index.html with html-webpack-plugin.

[hjhsalo]

This probably would help with cache invalidation problems, where clients (i.e. browsers) won't fetch a new copy of b2share-bundle.js after webpack build.