[WIP] Add subscribe/unsubscribe feature.

[mounilKshah]

What does this PR do? This PR is to add subscribe-unsubcribe toggle for all streams present in the stream. Currently, the commit in the PR only works for a single stream. Partial fix for #570

Discussion topic: zulip-terminal>Add provision to subscribe to streams

Tested?

• [ ] Manually
• [ ] Existing tests (adapted, if necessary)
• [ ] New tests added (for any new behavior)
• [ ] Passed linting & tests (each commit)

Commit flow

• Popup for subscribe/unsubscribe toggle

Notes & Questions

• Works only for one stream and it has to be in the same session. Needs to be extended to all streams
• Does not update the stream view, nor the messages in the MiddleColumn

Visual changes

• To be added