Fix timing issues on convertion migration award emoji

What does this MR do?

Fixes timing issues while migrating the data from the notes table to the award_emoji table. Also, it adds a down migration for some of the data.

Are there points in the code the reviewer needs to double check?

I've tested the queries myself on PG only, as at of this time I haven't got MySQL yet. Though there shouldn't be a problem here.

What are the relevant issue numbers?

Closes #18385 (closed)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together