user attaches proofs and only sees their proofs

This features allows proofs to be assigned only to a specific user.

We’re currently using a hidden_field with user_id on `proofs#new`.
However, we’re not entirely sure if that’s best practices. Rails guides
says that nesting user, proofs, requirements is bad practice, so we
couldn’t do something like `form_for [@user, @proof, @requirement]`