Crash on narrows with inactive bots/deactivated users

[Ezio-Sarthak]

Steps to reproduce:

• Make a bot (potentially of any type)
• Start the conversation (a couple of messages, to help narrow to it later)
• Deactivate the bot (from web app :))
• Perform either narrowing or quote reply actions on that conversation in ZT

You'll encounter a traceback, with the error Key error: <bot-email>. The key error corresponds to Model.user_dict.

We currently don't support tracking users which are deactivated (including inactive bots). We do check that a narrow exists first in other cases (e.g., narrowing to stream). This might be an option for users too. We can also add a check based on the server response.

[neiljp]

This is likely blocked on handling events as users add and leave the organization, part of #993.