streams: Handle adding/removing stream events.

[Abhirup-99]

Fixes #816

[Abhirup-99]

@neiljp Sorry, the pr became a bit longer than I had anticipated it to be. This happened because of the addition of the tests, the incoming events from add_stream event is quite large so that primarily contributed to it, also handling of some race around conditions when the user is removed from a stream but is that stream is on view. I disabled some of the keypress events when that condition occurs. Any other thoughts/suggestions would be helpful.

[Abhirup-99]

@zulipbot add "PR needs review"

[neiljp]

@Abhirup-99 You replied to some points, but do you want to do some big refactoring first, or go for a simpler version? I know what I'd like to do for the refactoring, but we could get this fixed first.

[Abhirup-99]

@zulipbot add "PR needs review".

[Abhirup-99]

@zulipbot remove "PR needs update"

[Abhirup-99]

@zulipbot add "PR needs review"

[Abhirup-99]

@zulipbot remove "PR needs update"

[Abhirup-99]

@zulipbot add "PR needs review" remove "PR awaiting update"

[Abhirup-99]

@zulipbot remove "PR awaiting update"

[zulipbot]

Heads up @Abhirup-99, we just merged some commits that conflict with the changes your made in this pull request! You can review this repository's recent commits to see where the conflicts occur. Please rebase your feature branch against the upstream/main branch and resolve your pull request's merge conflicts accordingly.