feat: web client refresh

[ckerr]

This is a major overhaul of the web client.

User-visible highlights include:

• Mobile is now fully supported.
• Added fullscreen support on mobile.
• Better support for dark mode.
• Added mime icons to the torrent list.
• Improved theme consistency across the app.

Maintainer highlights include:

• Updated code to use ES6 APIs.
• No longer uses jQuery UI.
• No longer uses jQuery.
• Use Webpack to bundle the Javascript, CSS, and assets together -- the entire bundle size is now 68K gzipped.
• Added eslint / prettier / stylelint tooling.
• Uses torrent-get's 'table' mode for more efficient RPC calls.

[GreenLunar]

Please consider #1680

[GreenLunar]

Was /transmission/web/style/transmission/images/progress.png replaced by CSS?

If not, it will be nice to have it as CSS, for scalability.

[ckerr]

Was /transmission/web/style/transmission/images/progress.png replaced by CSS? If not, it will be nice to have it as CSS, for scalability.

Yes. It now uses linear-gradient