notif: Show organization name, not URL, in collapsed notification

[gnprice]

See chat thread.

This will be a followup to:

• notif: Show organization URL in collapsed notification #569

After #569 is done, the main work that's required in order to show the name instead of the URL is that we need to keep the name around in our local database. (It's not included in the notification blobs from the server.) So the following issue is a prerequisite:

• Track each realm's name and icon in database #1036

[chrisbobbe]

From beta feedback today:

At this moment it's hard to see how notifications differs from realm to realm. All I can see in notification center is https://chat. or something similar.

This feature should help with that experience.

[gnprice]

It turns out the organization name may in fact be included in those notification blobs from the server. We learned that in the course of starting to document this part of the Zulip API:
#api design > E2EE push notifications settings @ 💬
#api design > E2EE push notifications settings @ 💬

In that case this issue doesn't have any prerequisites, and probably won't be hard to do. Bumping it to a nearer-term milestone.