about_popup: Terminal size added.

[Swarnim114]

closes #1536

What does this PR do, and why?

Adds a Terminal size label under the "Detected Environment " in About popup

External discussion & connections

• [ ] Discussed in #zulip-terminal in topic
• [x] Fully fixes #1536
• [ ] Partially fixes issue #
• [ ] Builds upon previous unmerged work in PR #
• [ ] Is a follow-up to work in PR #
• [ ] Requires merge of PR #
• [ ] Merge will enable work on #

How did you test this?

• [x] Manually - Behavioral changes
• [x] Manually - Visual changes
• [x] Adapting existing automated tests
• [ ] 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

• [ ] It is a minimal coherent idea
• [ ] It has a commit summary following the documented style (title & body)
• [ ] 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
• [ ] It flows clearly from a previous branch commit, and/or prepares for the next commit

Visual changes

[zulipbot]

Hello @Swarnim114, it seems like you have referenced #1536 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 #1536..

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.

[zulipbot]

ERROR: Label "Pr need review" does not exist and was thus not added to this pull request.

[Swarnim114]

@zulipbot label "PR needs review"

[Swarnim114]

@rsashank, I apologize for the confusion. This will be my final pull request for this issue. Thank you for your patience.This is the same code from before,