narrow: Update hash for moved message even if hash change in progress.

[laurynmm]

Adds a new trigger string to use for narrow.activate opts when it is called due detecting a message move for the targeted message ID: "retarget message location".

Updates narrow.save_narrow and narrow.hashchange to accept the opts.trigger as a parameter so that, even if the narrow was started via a hash change in the web app, the URL and browser history is updated for the current location of the targeted message.

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Self-review checklist

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

Communicate decisions, questions, and potential concerns.

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• Automated tests verify logic where appropriate.

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Completed manual review and testing of the following:

• Visual appearance of the changes.
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• Corner cases, error conditions, and easily imagined bugs.

[timabbott]

The strategy makes sense to me.

[timabbott]

Merged, thanks @laurynmm!