notif: Show time in collapsed notification

[gnprice]

See chat thread.

To implement this, I believe what's needed is to make a "summary notification" and put the information there. See the notifications code in zulip-mobile (in Kotlin, under android/) for comparison.

We already show the time on individual notifications — just not on the collapsed notification.

This might get done at the same time as the following related issue:

• notif: Show organization URL in collapsed notification #569