chore: API fixes (#3014)

- Minor API fixes
- Configuration screen for LINE inbox

app/javascript/dashboard/i18n/locale/en/inboxMgmt.json

@@ -195,6 +195,10 @@
         "SUBMIT_BUTTON": "Create LINE Channel",
         "API": {
           "ERROR_MESSAGE": "We were not able to save the LINE channel"
+        },
+        "API_CALLBACK": {
+          "TITLE": "Callback URL",
+          "SUBTITLE": "You have to configure the webhook URL in LINE application with the URL mentioned here."
         }
       },
       "TELEGRAM_CHANNEL": {