Linkify links on Hangouts-side

[dehnert]

Hangouts has a weird markup syntax. Since we claim to know how to do it, but don't mark links as links, Hangouts doesn't render it as a link. We should actually mark links as such.

[dehnert]

Note that there's a from_str method -- it's plausible we want to just feed in the Zulip markdown?

[dehnert]

Fixed in commit 37a0cb4. (Note that links with custom text don't work -- that's a Hangouts limitation, I think, although we could convert them to the custom text and a clickable URL in parens or something.)