Add handling for User update events

[mounilKshah]

What does this PR do? The PR is for handling any changes made with respect to the users' meta data ie, meta data of the current user and that of other users.

Fixes #988

Tested?

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

Commit flow

• Only one commit currently: Updates data structures in the codebase as per the events received.

Notes & Questions

• Yet to imbibe UI changes for the user events.

[zulipbot]

Hello @mounilKshah, it seems like you have referenced #988 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 #988..

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.

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[zulipbot]

Heads up @mounilKshah, we just merged some commits that conflict with the changes you 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.

[neiljp]

@mounilKshah Thanks for the updates here! I just merged the first two commits of this manually with some minor adjustments to avoid the need to add the "email" field :tada: (last commit was https://github.com/zulip/zulip-terminal/commit/01b3853c45260bcc5dc0174d6a028e8d9c03fc50)

I think we'll likely need to do quite a lot of refactoring before other updates of this form, so the user updates aren't as critical right now.