FormFyxer field normalizing doesn't always create valid identifiers

[nonprofittechy]

We should filter the labels using the same method Docassemble does--which is werkzeug.secure_filename, and also use label.isidentifier() where it's appropriate as a second check.

Look, e.g., at what it does with this MA form: https://suffolklitlab.org/form-explorer/form/MA/5e7716400e4867163f60c513fa29e9a9.html

The normalizer creates field names like s_15[0] and s_19[0].