msglist: In keyword search narrow, highlight keywords in message content

[chrisbobbe]

Fixes #1692.

Still TODO:

• Tests (model and widget tests for the message list? maybe just model tests)
• Resolve a slightly annoying rebase conflict with Show message content, sender, and timestamp in message action sheet #1624

But otherwise I think this is ready.

—Oh no, hmm, the code-blocks issue #1695 might need to block this after all; it's ugly when there's a match in a code block:

Figma for what highlighting should look like: https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=10904-102304&m=dev

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cc @alya to look at the screenshots.