[WIP] Add subscribe/unsubscribe feature.

[mounilKshah]

Moved to #1266

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