Migrate internal code to use Direct Message terminology

[Ajnus]

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What does this PR do, and why?

Replaces instances of: private, privates, _ private, private _, PRIVATE, PRIVATES, Private , pm, pms, _ pm, pm _, PM, Pm (and derivatives, e.g: _is_private_message_to_self, private_msg_ids, private_msg_ids_by_user_ids, (...))

to Direct Message terminology, avoiding the test_model.py, model.py files, strings and other instances related to the API, for API consistency reasons, while "exploring other parts", such as replacing comments.

This PR consists of the - on-chat debated - first part of altering just internal code, not the second that focus on API related code.

External discussion & connections

• [x] Discussed in #zulip-terminal in #T1344 Migrate internal code to use Direct Message terminology
• [x] Fully fixes #1344
• [] Partially fixes issue #
• [x] Builds upon previous unmerged work in PR #1344
• [ ] Is a follow-up to work in PR #
• [ ] Requires merge of PR #
• [ ] Merge will enable work on #

How did you test this?

• [ ] Manually - Behavioral changes
• [ ] Manually - Visual changes
• [ ] 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)
• [x] It runs

Self-review checklist for each commit

• [x] 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

[neiljp]

@Ajnus Just a quick note re testing in general - these can all be run locally, as per the development notes in the README. In particular there is tools/check-branch, for the isolated commit check itself.

[zulipbot]

Hello @Ajnus, it seems like you have referenced #1344 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 #1344..

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.

[Ajnus]

Hello @Ajnus, it seems like you have referenced #1344 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 #1344..

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.

Doesn't this breaks @github-actions Linting & tests / Lint - Commit text format (gitlint) (pull_request) ?

[Ajnus]

@Ajnus Just a quick note re testing in general - these can all be run locally, as per the development notes in the README. In particular there is tools/check-branch, for the isolated commit check itself.

Can't really. Even after make test

[timabbott]

@Ajnus the commit history needs to be cleaned up before this will be ready for review. Check out our GitHub guide and commit guidelines for more details.

[Ajnus]

@timabbott check the zulip chat please.