Fix prefer-template eslint rule in ES6 files

[MAILPOET-1139]

assets/js/src/form/fields/text.jsx

@@ -22,7 +22,7 @@ const FormFieldText = React.createClass({
           : false
         }
         name={this.props.field.name}
-        id={'field_' + this.props.field.name}
+        id={`field_${this.props.field.name}`}
         value={value}
         placeholder={this.props.field.placeholder}
         onChange={this.props.onValueChange}