To add a new rate limit or an application setting we should use a JSONB
column to store the new setting. It has the following advantages:

1. Prevents the `application_settings` from getting wider. Postgres has
   a limit of 1600 columns.
2. We can continue using the rails validations.
3. Store different data types.
4. Adding a new setting to an existing column won't a review from the
   database team.