[PLT-8173] Add username and profile picture to webhook set up pages (#8002)

i18n/en.json

@@ -1424,6 +1424,10 @@
     "id": "api.incoming_webhook.disabled.app_error",
     "translation": "Incoming webhooks have been disabled by the system admin."
   },
+  {
+    "id": "api.incoming_webhook.invalid_post_username.app_error",
+    "translation": "Invalid username."
+  },
   {
     "id": "api.ldap.init.debug",
     "translation": "Initializing LDAP API routes"
@@ -4978,6 +4982,14 @@
     "id": "model.incoming_hook.id.app_error",
     "translation": "Invalid Id"
   },
+  {
+    "id": "model.incoming_hook.username.app_error",
+    "translation": "Invalid username"
+  },
+  {
+    "id": "model.incoming_hook.post_icon_url.app_error",
+    "translation": "Invalid post icon"
+  },
   {
     "id": "model.incoming_hook.team_id.app_error",
     "translation": "Invalid team ID"