Issue #1344 (terminal cant find @issue#1344 for cloning and local testing)

[Ajnus]

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
• Fully fixes #
• [ x] Partially fixes issue TRACKING: Migrate internal code to use Direct Message terminology zulip/zulip-terminal#1344
• [x ] Builds upon previous unmerged work in PR TRACKING: Migrate internal code to use Direct Message terminology zulip/zulip-terminal#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)

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