Source-folio-sequence uniqueness requirement

[dchiller]

This requirement is currently tested in the form_valid method of the ChantCreateView. It is not tested at all for any other chant editing methods (either the Admin panel or the ChantEditView), and indeed, this method is the wrong place for this kind of validation (the validation should occur on the form and not on the view). Eventually, this would be a good place for a uniqueness constraint on the model itself, but there is messy data currently.

We'll use #1714 to fix this data and then add a model uniqueness constraint. In the meantime, I'll move these validations to a form method.