DS-312 Prevent form submission if a non-required field fails validation.

## Changes
 - Prevents form submission if a filled-in `data-validate` field doesn't validate, even if it's not required. (So, on the registration form for example: phone could be blank and form would submit, but if the user has entered content into that field, it must validate). Previously only required fields blocked form submission.

Relevant JIRA ticket: [DS-312](https://jira.dosomething.org/browse/DS-312)

## How do I test?
Pull down this branch, drop it in your `bower_components/neue` directory in the main app, and take it for a spin.