Generate narrow links using "channel" operator, not "stream"

[laurynmm]

This is one of the final, follow-up tasks for #28468 (Rename "stream" to "channel").

When we generate narrow links in the web app (e.g. for quote-and-reply) and in emails (e.g. missed message notifications), we should start using the "channel" operator instead of the "stream" operator.

This is blocked until enough time has passed for folks to upgrade to the zulip-mobile client version that supports/recognizes the "channel" operator in these links.

CZO thread