PLT-2170 Send payload in application/json for outgoing webhooks (#3160)

* Send payload in application/json for outgoing webhooks

The Add outgoing webhook UI now has a 'Content-Type' field that allows
to choose between application/x-www-form-urlencoded and
application/json. All outgoing webhooks created before this change will
be considered as x-www-form-urlencoded. There's also a minor change in
the way the outgoing webhook summary is displayed: the 'Callback URLs'
label was missing.

* Fix JS formatting errors

* Increase ContentType field length to 128

webapp/sass/routes/_backstage.scss

@@ -220,6 +220,7 @@ body {
     }
 
     .item-details__description,
+    .item-details__content_type,
     .item-details__token,
     .item-details__trigger-words,
     .item-details__url,