chore(web-console): Import: Handle duplicate files

[insmac]

This PR adds handling of adding multiple files of the same name to the upload queue. The current version is buggy and does not inform the user if a duplicate has been detected.

The new behavior is as follows:

• Duplicate file names are allowed as long as the target table name is unique
• If duplicates have been found, a relevant error message is displayed
• Duplicates are filtered out before adding the batch to the queue.

Side changes:

• Paste event handler has been moved to Dropbox, so that the de-duplication logic is unified and displays the same error message every time for file browsing, drag & drop and paste.

Screen recording:

CleanShot.2023-05-31.at.16.34.58.mp4