Trip null characters from input fields in the admin pages

[marcospri]

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:

• Add a way to trim whitespace characters marshmallow-code/marshmallow#1391
• https://stackoverflow.com/questions/38883476/how-to-remove-those-x00-x00