Fix Mutation Event issues by using Mutation Observer instead

[ObadaS]

@Didayolo

This is a fix of the Chrome issue reported here : codalab/codalab-competitions#3503

It is based on bradleyg@d7d4d29 with some modification since this code alone didn't work.

To test this code on Codalab:

• Clone codalab into a new folder
• Switch branch to chromeFix
• Connect a working MinIO to codalab through the .env
• Build the stack docker compose up -d
• Open Chrome, go to localhost then upload a competition
• Make a submission on this new competition. It should work fine. The error in the Chrome dev console should also be gone

I don't have any large submission to test so it would be nice if this can be tested too.