[flag] Add flag editors table

[skarya22]

Brief summary of changes

• Introduce a flag_editors table that tracks who has edited an instrument
• Removes UserID from flag as it is replaced with the first entry of flag_editors
• Removes User1 and User2 from conflicts_resolved as they are misleading columns, and also not useful now that UserID is gone from flag

Testing instructions (if applicable)

1. Edit an instrument
2. See that your UserID was added to the flag_editors table for that commentID
3. Try starting a visit for a participant and see that the flag entry was added, and also to flag_editors

CCNA OVERRIDE PR