Linkify telephone numbers

[gnprice]

In some messaging apps on mobile (including Android Messages, iMessage, and Signal), if some text in a message looks like a phone number, it turns into effectively a link for the phone number.

A user requested the same feature for Zulip today in chat.

The behavior seen in other apps is a bit different between iOS and Android: see details, with screenshots, in the chat thread.

It's not yet clear just how we'd want this to work. One possibility is that the server would linkify these when the message is sent, like we do for bare URLs. That'd mean it appears as a link on all platforms, even desktop; might be annoying for many users, but at least some users want that behavior.

[chrisbobbe]

Let's look into making this conditional on whether the device actually has an app that can open tel: links.

We should be able to check that with a query to url_launcher.canLaunchUrl. Looking at that API and the linked platform APIs, I think we'd want to do just one query, on startup, and remember the result.