Design: Mobile multiserver support and sidebar improvements

Investigate how we can support multiple servers on the mobile app. Some considerations:

1. How do you switch between servers?
2. How do you add new servers?
3. How do you indicate unreads/mentions between servers?

4. How do you indicate and switch servers in the Share Extensions?

Here’s the doc with expected:
https://docs.google.com/document/d/1dxQQmFYIzXBv0G6TvnWhziu_PHTw_nPMeVGw2JbO3Zc/edit#