Add remove_reaction for bot handlers

[PIG208]

Currently, the bot is able to add a reaction conveniently using react(message, emoji_name). Similarly, we can also wrap the API call for removing a reaction for our bot handler. We will need to modify the BotHandler protocol and our implementations for ExternalBotHandler and EmbeddedBotHandler (in the Zulip repository) for this change.

A possible direction for us here might be having the bothandler to support most of the things that a bot is "supposed" to be able to do, without having the developer worries about any specific REST API.