markdown: Remove most of Twitter integration.

[timabbott]

Twitter removed their v1 API. We take care to keep the existing cached results around for now, and to not poison that cache, since we might be able replace this with something that can still use the existing cache.

Fixes:

Screenshots and screen captures:

Self-review checklist

• Self-reviewed the changes for clarity and maintainability
  (variable names, code reuse, readability, etc.).

Communicate decisions, questions, and potential concerns.

• Explains differences from previous plans (e.g., issue description).
• Highlights technical choices and bugs encountered.
• Calls out remaining decisions and concerns.
• Automated tests verify logic where appropriate.

Individual commits are ready for review (see commit discipline).

• Each commit is a coherent idea.
• Commit message(s) explain reasoning and motivation for changes.

Completed manual review and testing of the following:

• Visual appearance of the changes.
• Responsiveness and internationalization.
• Strings and tooltips.
• End-to-end functionality of buttons, interactions and flows.
• Corner cases, error conditions, and easily imagined bugs.

[kou]

@timabbott I don't know why but my Zulip instance still works well with the original Twitter integration code.
Can we revert the removal until all Zulip instances don't work with the original Twitter integration code? Because tweet preview feature is useful.
(I'm OK that I need to set a variable explicitly (like USING_PGROONGA = True for PGroonga) to enable Twitter integration with the original Twitter integration code. I'm using Zulip 7.1 with local modification for now.)

[n0099]

@kou How did you patch the zulip 7.1? Does it apply to the latest zulip 8.0 on the zulip-cloud-current branch?

[kou]

I changed local deployed Zulip source directly.

FYI: I don't need this now because Twitter API 1.1 was stopped. https://github.com/zulip/python-zulip-api/blob/main/zulip/integrations/twitter/twitter-bot doesn't work.