Increase character limit for creator field in upload dataset API

[debayan]

The current 'creator' field in the upload API has a character limit of 128. Most dataset papers have multiple authors and adding their names can easily cross the current limit. It would be nice to have a much larger limit for this field.

[joaquinvanschoren]

Moving this to the OpenML API repo.
@PGijsbers: good to keep in mind for the new version of the API.