Validate new Learners and Instructors

Validating single entries on the Instructor and Learner tabs - with tests
Validating CSV uploads and displaying errors by line for these tabs - no tests, couldn't get the file upload to work correctly.


Need to confirm that these messages are correct:
- [ ] Last Name is Required
- [ ] First Name is Required
- [ ] Email is Required
- [ ] Campus ID is Required
- [ ] Email address is not valid
- [ ] Campus ID is not long enough
- [ ] Campus ID is too long
- [ ] Incorrect Number of Fields
- [ ] A user with this email is already in the system
- [ ] Email address in the file more than once

Fixes #561