App should handle Login form validation in a more React-friendly way

[francisli]

The migration of the Login/Forgot Password/2FA/Reset Password flows into the SPA resulted in some old JavaScript DOM related code being reused- particularly, in client/src/Auth/Components/helperFunctions.js.

These helper functions are being used to perform field validation, manipulating the DOM directly.

I think this should all be refactored into a more React-friendly implementation.

[fwextensions]

I haven't investigated this code, but it might be possible to use the native HTML input validation as well.