[WIP] Migrate message['subject'] to message['topic']

[mounilKshah]

What does this PR do? This commit migrates the ZT to newer API elements. Here, the migration is from message['subject'] to message['topic']. Partial fix for #965

Discussion: #zulip-terminal>Update to modern API elements (support v2.1) #T965

Tested?

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

Commit flow

• First commit updates message element from subject to topic
• Second commit refactors the tests to accommodate this migration

Notes & Questions

• Fixes item 4 on #965

[zulipbot]

Hello @zulip/server-refactoring members, this pull request was labeled with the "area: refactoring" label, so you may want to check it out!

[neiljp]

@mounilKshah I left some answers in the stream; do you need further feedback before the tests are updated?

[neiljp]

@mounilKshah Thanks for investigating this!

I left a message in the stream, but for other readers also:

• The API relating to migration of the subject field to topic is the only (or almost only) renaming of this kind which it seems has not yet occurred.
• Since the only current commit in this PR relates to this, I propose we hold off on this migration, since this changes our internal data structures unnecessarily.
• Another part related to this migration is when sending messages via Composition objects, which supported topic since 2.0.0; let's redirect that effort into another PR.

[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.