Consolidate callback forms and convert them to gov uk frontend

We had two identical callback form classes. One for delivery callbacks
and one for inbound sms callbacks. Since they did not differ I consolidated
them into one CallbackForm class that they both inherited from previously.

I also substituted field classes for this form with new fields
that cooperate with gov uk frontend.

app/main/views/api_keys.py

@@ -16,12 +16,7 @@ from app import (
     service_api_client,
 )
 from app.main import main
-from app.main.forms import (
-    CreateKeyForm,
-    GuestList,
-    ServiceDeliveryStatusCallbackForm,
-    ServiceReceiveMessagesCallbackForm,
-)
+from app.main.forms import CallbackForm, CreateKeyForm, GuestList
 from app.notify_client.api_key_api_client import (
     KEY_TYPE_NORMAL,
     KEY_TYPE_TEAM,
@@ -195,7 +190,7 @@ def delivery_status_callback(service_id):
         else '.api_integration'
     )
 
-    form = ServiceDeliveryStatusCallbackForm(
+    form = CallbackForm(
         url=delivery_status_callback.get('url') if delivery_status_callback else '',
         bearer_token=dummy_bearer_token if delivery_status_callback else ''
     )
@@ -255,7 +250,7 @@ def received_text_messages_callback(service_id):
         return redirect(url_for('.api_integration', service_id=service_id))
 
     received_text_messages_callback = get_received_text_messages_callback()
-    form = ServiceReceiveMessagesCallbackForm(
+    form = CallbackForm(
         url=received_text_messages_callback.get('url') if received_text_messages_callback else '',
         bearer_token=dummy_bearer_token if received_text_messages_callback else ''
     )