Add key to the Comment field of feedback_mri_comments to improve querying time

[cmadjar]

Brief summary of changes

Adding this index improved querying from 2 minutes to 3 seconds on the feedback_mri_comments on HBCD (which contains 12063735).

Testing instructions (if applicable)

1. run the SQL patch and make sure the index was added to feedback_mri_comments
2. source the default schema and make sure the index was created in feedback_mri_comments

[driusan]

@cmadjar This is failing when sourcing the schema because of the SQL error:

"ERROR 1170 (42000) at line 2265: BLOB/TEXT column 'Comment' used in key specification without a key length"

I'm not really sure why that's happening with the API tests and not the other ones, but it seems to be a real error and not a sporadic failure.