channel/edit - Edits an existing channel.

Method parameters:
id - The ID of the channel to edit.
channel_name - The new name of the channel.
public - Whether the channel should be marked as public.
receive_notifications - Whether the channel owner should receive notifications.

Returns a JSON document of the following format:
<boolean> - Always true.

Expected errors:
CHANNEL_NOT_FOUND - A channel with the ID doesn't exist.
ENTER_CHANNEL_NAME - The new name is empty.
INVALID_CHANNEL_NAME - The new name is invalid.
CHANNEL_NAME_TOO_SHORT - The channel name is shorter than 6 characters.
CHANNEL_ALREADY_EXISTS - A channel with the same name already exists.