boxes/views: Adopt urwid_readline for all editors.

[Niloth-p]

What does this PR do, and why?

Adopts urwid_readline.ReadlineEdit for all the editors in Zullip Terminal.
The panel search boxes are the only remaining editors not using urwid_readline.ReadlineEdit.

External discussion & connections

• Discussed in #zulip-terminal in Readline shortcuts in search boxes
• Fully fixes #
• Partially fixes issue #
• Builds upon previous unmerged work in PR #
• Is a follow-up to work in PR #
• Requires merge of PR #
• Merge will enable work on Introduce help context #1484

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

[neiljp]

@Niloth-p Thanks for the small but effective PR 👍

Since we have the message search already also using a ReadlineEdit, I didn't see an issue with merging this now, since the help is already slightly inconsistent. However, I'd like to see a prompt update to the help category related to only this, so that we ensure this doesn't get lost. Other help category updates are important, but potentially independent.