Generate narrow links with "channel", not "stream", where supported

[gnprice]

This is the zulip-flutter counterpart to the following zulip-mobile issue:

• Generate narrow links using "channel" operator, not "stream", if server is recent zulip-mobile#5861

We should do the same thing here.

Like in zulip-mobile, it won't yet be time to do this until a zulip-mobile release with zulip/zulip-mobile#5860 (to recognize the same "channel" operator) has been out to users for some weeks.

Related server/web issues:

• Umbrella issue: Rename "stream" to "channel" zulip#28468
• Specific corresponding issue: Generate narrow links using "channel" operator, not "stream" zulip#30385

[gnprice]

In order for this to really work properly, it'll need:

• Update database with feature level etc. on each register #458

Without that, for a user who's been using this app with e.g. chat.zulip.org since before the relevant feature level, and hasn't reinstalled the app since then, the app will continue to think the server has an older version and so it won't activate this feature.

[gnprice]

The server and web app won't be generating these links for several more months, for Server 10 rather than Server 9, in order to leave time for mobile users who don't get prompt updates to update to versions that support the new form of URL. See chat thread.