Make file upload reliable

[exalate-issue-sync]

Description

User Stories

• As a user I want that the uploadprocess works in the background so that I can continue with my work without interference.

• As a user who wants to upload one or many files, I want to be able to pause and resume uploads so that I have control over my bandwidth (and dont have a laggy videochat while uploading).

Value

• control over uploads
• groundwork to improve upload visualisation (both style and performace)

Acceptance Criteria

• check if uppy is accessible; if not, document findings, maybe we contribute to uppy to make it a11y (different story)

---

• show "uppy status bar" as floating dialog in the bottom right corner (c.f. google drive upload)
• starting an upload can be done via drag n drop into the files list or via upload-button(s)
• needs also to be implemented in the "public files drop link" (shows dashboard component)
• reasoning, why we prefer uppy (at the moment); document reasoning.
• bonus 🍰 - if possible: only have 1 upload button instead of dedicated file- and folder-upload buttons
• nice to have: adapt to oc-theming

Definition of done

• Functional requirements
  [ ] functionality described in the user story works
  [ ] acceptance criteria are fulfilled
• Quality
  [ ] codre review happened
  [ ] CI is green
  [ ] critical code received unit tests by the developer
  [ ] automated tests passed (if automated tests are not available, this test needs to be created and passed
• Non-functional requirements
  [ ] no sonar cloud issues

[pascalwengerter]

Whoever works on this please take a look at the linked closed PRs in #5371 regarding "improving the upload handling with TUS"