Add support for all streams (more general accessor methods)

[theViz343]

What does this PR do, and why?

This PR adds support for unsubscribed streams and never_subscribed streams in ZT. It creates new data structures and general accessor methods to access them. This PR is needed for further work on reducing stream related issues and adding support for stream (un)subscription. This PR is based heavily on the initial commits of #1408 with some slight changes and the generalization of the accessor methods.

Outstanding aspect(s)

• Does the typing for the accessor methods look fine?

External discussion & connections

• [x] Discussed in #zulip-terminal in Adding streams throws key error
• [x] Fully fixes #816
• [ ] Partially fixes issue #
• [ ] Builds upon previous unmerged work in PR #
• [ ] Is a follow-up to work in PR #
• [ ] Requires merge of PR #
• [x] Merge will enable work on #1387

How did you test this?

• [x] Manually - Behavioral changes
• [x] Manually - Visual changes
• [x] Adapting existing automated tests
• [x] Adding automated tests for new behavior (or missing tests)
• [ ] Existing automated tests should already cover this (only a refactor of tested code)

Self-review checklist for each commit

• [x] It is a minimal coherent idea
• [x] It has a commit summary following the documented style (title & body)
• [x] It has a commit summary describing the motivation and reasoning for the change
• [ ] It individually passes linting and tests
• [ ] It contains test additions for any new behavior
• [x] It flows clearly from a previous branch commit, and/or prepares for the next commit

Visual changes

[zulipbot]

Hello @theViz343, it seems like you have referenced #816 in your pull request description, but you have not referenced them in your commit message description(s). Referencing an issue in a commit message automatically closes the corresponding issue when the commit is merged, which makes the issue tracker easier to manage.

Please run git commit --amend in your command line client to amend your commit message description with Fixes #816..

An example of a correctly-formatted commit:

commit fabd5e450374c8dde65ec35f02140383940fe146
Author: zulipbot
Date:   Sat Mar 18 13:42:40 2017 -0700

    pull requests: Check PR commits reference when issue is referenced.

    Fixes #51.

To learn how to write a great commit message, please refer to our guide.