Disable cycling to recipients when editing PM

[zormit]

What does this PR do, and why?

This is a rebase and simplification of #1280. It's the missing piece to resolve #774.

In comparison to the previous work

• heavily restructured the commits and refined/clarified the descriptions.
• I also decided to use a more "high level" approach to testing, testing less the internal calls, rather than the overall expected resulting structure.
• The error message when pressing tab I removed as well, because I thought it was not a real "error", but maybe it's more user friendly than noisy?

External discussion & connections

• [x] Discussed in #zulip-terminal in Disable PM recipient cycling on edit #T774 #T1280
• [x] Fully fixes #774
• [ ] Partially fixes issue #
• [x] Builds upon previous unmerged work in PR #1280
• [ ] 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
• [ ] Manually - Visual changes
• [x] Adapting existing automated tests
• [x] 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

• [x] It is a minimal coherent idea
• [ ] It has a commit summary following the documented style (title & body)
• [x] It has a commit summary describing the motivation and reasoning for the change
• [ ] It individually passes linting and tests
• [x] It contains test additions for any new behavior
• [x] It flows clearly from a previous branch commit, and/or prepares for the next commit