fix(files-sidebar): reduce initial loading size of Files sidebar

[Antreesy]

☑️ Resolves

• Changes:
  • Initial sidebar tab registration takes only a loading placeholder
  • When tab becomes active, component triggers async load of main content and replace placeholder
  • From that moment, all logic should remain the same, only main content will be used for tab mounting

🖌️ UI Checklist

🖼️ Screenshots / Screencasts

🏚️ Before	🏡 After
481 kB initial + 1.5 MB (chat + call)	41 kB initial + 1.2 MB (chat + call)

🚧 Tasks

• Check properly

🏁 Checklist

• 🌏 Tested with Chrome, Firefox and Safari or should not be risky to browser differences
• 🖌️ Design was reviewed, approved or inspired by the design team
• ⛑️ Tests are included or not possible
• 📗 User documentation in https://github.com/nextcloud/documentation/tree/master/user_manual/talk has been updated or is not required

[ShGKme]

Rebased onto main and resolved conflicts

[nickvergessen]

get in? backport?

[Antreesy]

get in? backport?

Tested once again, works correctly. Most of the JS code (1 Mb in dev) is loaded after sidebar is opened and DOMContentLoaded, so somewhat a boost. I'm fine with both options