Create `User.reset_password_changeset/2`

[begedin]

Problem

We need to define reset_password_changeset(struct, params) on User

• casts :password and :password_confirmation
• validates that the two are equal
• if valid, encrypts the password and puts it as :encrypted_password into the changeset
• if invalid, should add an error
• we can make use of User.put_pass_hash to achieve parts of this
• we probably need to add a virtual :password_confirmation field to the user schema

References

Progress on: #498