Apply local or global data transformations

[alexweissman]

Instead of "sanitization", we could allow Fortress to apply local or global data transformations, as discussed here: https://github.com/userfrosting/UserFrosting/issues/423

The understanding would be that pre-storage transformations of user data would be the exception, rather than the rule.

[alexweissman]

How about allowing an arbitrary series of transformations? I.e.:

transform: ['trim', 'purge']

or

transform: ['purify', 'trim']