Add more robust check for send form presence to acceptance tests [MQ-29]

There is a short delay between rendering of a heading and the form fields.
It is safer to wait for form itself before manipulating it.

assets/js/src/form/form.jsx

@@ -216,6 +216,7 @@ const Form = React.createClass({
               ? this.props.onSubmit
               : this.handleSubmit
           }
+          data-automation-id={this.props.automationId}
         >
           { errors }
 

assets/js/src/newsletters/send.jsx

@@ -349,6 +349,7 @@ const NewsletterSend = React.createClass({
         <Form
           id="mailpoet_newsletter"
           fields={fields}
+          automationId="newsletter_send_form"
           item={this.state.item}
           loading={this.state.loading}
           onChange={this.handleFormChange}