"I Call Your Name" or Added student name, birthday, and birthplace validations

Started rewriting the `show` and `update` methods within `enrollment_controller`. The new versions are currently written as `new_show` and `new_update` until they are complete from beginning to end (otherwise, this may interfere with UI development). To enable the new implementations, flip `EnrollmentController::USE_NEW` to `true` (just be aware it's WIP).

Changes include:

- Added validations for Student for `first_name`, `middle_name`, `last_name`, `birthday`, `birth_city`, `birth_state`, and `birth_country`.
- Added rough display of errors on `student_name` page (this is just to see that the errors are happening, this can be revised to be more aesthetically pleasing and more closely associated with the fields in question)
- Revised references to steps for the sign posts such that variables don't need to be evaluated each time `show` happens.
- Adds gem for date validations

This is working towards the BE tasks for #101 , #103 . 

@codesliced Ready for review.