Give option to open another org's `#narrow` link in-app

[chrisbobbe]

Currently if you tap a valid narrow link but it belongs to another org, we open it in the browser instead of in-app.

We got a request for this feature today: https://chat.zulip.org/#narrow/stream/48-mobile/topic/Bug.20reports/near/1479820

Quoting myself from there:

[W]e haven't built UI to ask if you want to switch to an account on that org and see the message in-app from there. If we added such a UI today, it would feel sluggish in the common case where you have two accounts in different orgs that you switch between regularly, since it would have to load data afresh when you switch from one to the other. In a future where we can maintain a connection to multiple accounts/orgs at a time (#5005), we can handle that case more gracefully.