Trip null characters from input fields in the admin pages

Currently we strip (python str.strip) all white characters on str fields but that doesn't remove all nullable characters leading to confusing results.

Create a new type of field we can reuse across all the admin pages that trims white characters and nullable ones.

See:

- https://github.com/marshmallow-code/marshmallow/issues/1391
- https://stackoverflow.com/questions/38883476/how-to-remove-those-x00-x00