Validation in check number form

[mprat]

Can we add validation in the check number form for returns? The backend only accepts a field of length 16, but deskworkers sometimes add longer text, throwing an error in the backend and (1) throwing a Sentry error that I can see, (2) I don't know what appears in the UI for deskworkers.

Can we limit that form to only take up to length 16 characters (https://github.com/mitoc/gear-db-django/blob/069edbd4ea8996bf1a2a31e19764d096d6288111/geardb/models/models.py#L716)?