Make it easier for Alt UIs to use translation files

[ghost]

Suggestion

1. Document the steps needed to compile src/webui/www/translations/*.ts files to *.qm so they can be used for Alternative Web UIs (obtain Qt Linguist, File -> Open, select all the files and do File -> Release all; preferably first do python tstool.py)
2. Get rid of pointless "is-alternative-web-ui" conditional checks which don't allow translation for alternative web UIs (e.g. https://github.com/qbittorrent/qBittorrent/blob/master/src/webui/webapplication.cpp#L529)
3. Bundle .qm files as artifacts on Github, so people don't have to download Qt apps (or leave them in translations).

Use case

Make it possible to develop and use Alternative Web UI without much effort and recompilation.

Extra info/examples/attachments

Relevant issues #18602 and #18820 (where these shortcomings haven't been made clear by maintainers)

[luzpaz]

Related to #18405

[glassez]

@stacksmash76
Could you read #17616 (comment) and below? It seems that @Chocobo1 did not take into account the fact that the alternative web UI feature is often used to use a slightly modified original web UI.

[ghost]

Thank you both for giving some context.

Indeed, @glassez you're completely right. I would personally make a PR for everything I listed above, as long there is a consensus between maintainers on how it should be done (point 2, to be exact). My current idea is to just make the HTML translation configurable (opt-in) via the UI. Do let me know whether you're okay with this.