Rename snake case form fields

[gsidebo]

Reference: https://github.com/mitodl/mitxonline/blob/main/static/js/components/forms/ProfileFormFields.js - LegalAddressFields and ProfileFields

The legal address form fields have snake case names that match the names and structure of the API results we get from the profile/legal address endpoint. Our form field names should all be camel case (see: static/js/components/forms/ChangeEmailForm.js) to match our general JS variable naming scheme, and the API results should be mapped appropriately to those fields. We also probably don't need to namespace the field names (e.g.: name="legal_address.first_name" --> name="firstName").

I noticed this when reviewing #161. That PR appends "Error" to the end of form field names to create id values, and there was no graceful way to do that given our dot-and-snake-case form field names