Unify confirmation popups into popup framework

[neiljp]

What does this PR do, and why?

This incrementally improves the popup code until a point where the confirmation popups can be handled in the same way as others and improved upon:

• extract the border styling from core.py and into a new PopUpFrame class in view.py
• extend PopUpFrame to handle title-less cases (ie. for confirmation popups)
• simplify confirmation popups to not support locations other than center, easing integration
• migrate ConfirmationPopUpView from an Overlay to a Frame, and other changes to enable use of show_pop_up just like other popups

Other than simplifying the code, this also results in confirmation popups acquiring a solid edge, as with regular popups.

Outstanding aspect(s)

• Last commit needs more consideration
• Nesting of popups into frame suggests frame could be an urwid container/decorator class
• Confirm ability to style popup borders
• Select appropriate popup borders for different confirmation popups?

External discussion & connections

• Discussed in #zulip-terminal in topic
• 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 #

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

Visual changes

[zulipbot]

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