compose: Make conversation arrow double as a go-to-conversation button.

[N-Shar-ma]

The chevron arrow icon before the topic / dm user field now also acts as a go to conversation button. Whenever the functionality is available, the plain icon changes to look and behave like a button. The old go to conversation button on the right of the field is removed.

Fixes: #28697.

(Alternative to #28915)

Screenshots and screen captures:

When the go to conversation functionality isn't available:
Light:

Dark:

When it is available:
Light:

Dark:

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.

[zulipbot]

Hello @zulip/design members, this pull request was labeled with the "redesign" label, so you may want to check it out!

[alya]

Works for me in manual testing! Posted a UX question on CZO.

[timabbott]

Test-deploying on chat.zulip.org.