Add another class for bot type as embedded bot, for bots that run directly
on Zulip server. This class uses internal 'internal_send_messages' from
actions.py instead of using the 'send_message' function from Zulip Client
class.
We haven't gone ahead with creating an abstract superclass right away.
We just have two versions for now, it would be easier to iterate a little
more on the interfaces, and then only add the superclass when we're ready
to lock down the interface.
This change is done for better understanding of the class functionality
from its class name. Now there will be 3 different classes for bots,
namely 'ExternalBotHandler', 'FlaskBotHandler' and 'EmbeddedBotHandler'.
