fix(billingServices): cancel btn does not validate

This commit tells angular not to validate the form when the cancel
button is pressed.

Closes #700.

client/src/partials/billing_services/modal.html

@@ -122,7 +122,7 @@ <h4>{{ BillingServicesFormCtrl.title | translate }}<span ng-show="BillingService
       {{ "FORM.BUTTONS.SUBMIT" | translate }}
     </bh-loading-button>
 
-    <button class="btn btn-default" data-method="back" ng-click="BillingServicesFormCtrl.dismiss()">
+    <button type="button" class="btn btn-default" data-method="back" ng-click="BillingServicesFormCtrl.dismiss()">
       {{ "FORM.BUTTONS.BACK" | translate }}
     </button>
   </div>