shorten Username isLong and isShort
validate channel params in a common function
use common function for validating channel name
validate channel name before subscribing to a public channel
check channel name before posting notification or subscribing to public channel
update api doc: add ENTER_CHANNEL_NAME and INVALID_CHANNEL_NAME errors
return INVALID_RECIPIENT_USERNAME error from send and sendExisting api methods
renamed request_received_user to require_received_user
renamed request_tags to require_tags