Changes to POST /channels/:channel_id/followers

added the non-success response with the status 429

added the pattern ^(0|[1-9][0-9]*)$ to the request property webhook_channel_id

the channel_id response's property pattern ^(0|[1-9][0-9]*)$ was added for the status 200

the webhook_id response's property pattern ^(0|[1-9][0-9]*)$ was added for the status 200