[ZD#732337] improve form validation

The form didn't validate all of the required fields so we were getting ajax failures on ticket creation. I updated the validation code to apply to all fields with a required property.

#### Risks:

- If the user doesn't autocomplete the requester email, the requester name remains a required field.

#### References:

- [ZD#732337](https://support.zendesk.com/agent/#/tickets/732337)

![linked-ticket](https://cloud.githubusercontent.com/assets/1010128/3126035/e99ce86a-e795-11e3-890e-7570a9d54f5b.gif)


/cc @maximeprades