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/templates/views/api/callbacks/received-text-messages-callback.html

@@ -21,17 +21,16 @@
       </p>
 
       {% call form_wrapper() %}
-        {{ textbox(
-          form.url,
-          width='1-1',
-          hint='Must start with https://'
-        ) }}
-        {{ textbox(
-          form.bearer_token,
-          width='1-1',
-          hint='At least 10 characters',
-          autocomplete='new-password'
-        ) }}
+      {{ form.url(param_extensions={
+        "classes": "govuk-!-width-full",
+        "hint": {"text": "Must start with https://"}
+      }) }}
+
+      {{ form.bearer_token(param_extensions={
+        "classes": "govuk-!-width-full",
+        "hint": {"text": "At least 10 characters"},
+        "autocomplete": "new-password"
+      }) }}
         {{ page_footer('Save') }}
       {% endcall %}
     </div>