channel/subscribed/subscribe - Subscribes the user to a public channel.

Method parameters:
channel_name - The name of the public channel to subscribe to.
receive_notifications - Whether the subscriber should receive notifications.

Returns a JSON document of the following format:
<number> - The ID of the newly subscribed subscribed channel.

Expected errors:
ENTER_CHANNEL_NAME - The channel name is empty.
INVALID_CHANNEL_NAME - The channel name is invalid.
CHANNEL_NOT_FOUND - A channel with the name doesn't exist.
CHANNEL_NOT_PUBLIC - The channel is not public.
CHANNEL_IS_OWN - You are the owner of the channel.
ALREADY_SUBSCRIBED - You are already subscribed to this channel.