fix(files): Allow to drag and drop new files also on empty directories

[susnux]

• Resolves: [Bug]: Drag and drop upload in sub directory: TypeError: this.$refs.table is undefined #41534
• Resolves: [Bug]: No user error for failed drag-and-drop or file upload error #41536
• Resolves: [Bug]: drag'n'drop zone only appears when there is a readme file in the folder. #41774

Summary

1. Move the drag and drop from the files table to the files list, to allow drag and drop upload in empty folders (where no table is shown but an empty content message). Also unregister the drag and drop listeners on component destroy, which was causing errors like this.$refs.table undefined.
2. Fix the missing error message for drag and drop uploads on failure.
3. Cleanup not used styles from the drag and drop notice.

Videos

Before

See that it is not possible to drop in empty folders

vokoscreenNG-2023-11-23_14-49-24.mp4

After

See that dropping is now possible in empty folders

vokoscreenNG-2023-11-23_14-48-17.mp4

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[susnux]

/backport to stable28

Resolved the comments

[AndyScherzinger]

/compile amend /

[susnux]

/backport to stable28