Fix real-time username/email formatting and OL availability validation

[rebecca-shoptaw]

Describe the problem that you'd like solved

There is good existing code to real-time validate username/email formatting and OL availability, which hasn't been running because of a tiny typo in the code.

This PR fixes that typo and also DRYs up a lot of unnecessarily duplicated and difficult to read logic in realtime_account_validation.js.

Proposal & Constraints

The fix involves just changing !value === "" to value !== "" in a few places, and the DRYing up should be quite simple -- will involve making renderError and clearError functions to be reused for email, username, and password.

Since the naming is very consistent for the inputs in the registration form (the only place this is currently used), we should be able to pass in a simple string, i.e. "emailAddr", to generate #emailAddr and #emailAddrMessage and label[for="emailAddr"] where needed, but I'll be sure to make a note re: the dependence on the current naming conventions.

Additional context

Stakeholders

@cdrini @jimchamp