Track each realm's name and icon in database

[gnprice]

This is a prerequisite for a couple of issues:

• notif: Show organization name, not URL, in collapsed notification #570
• Account-switcher modal #1038

We'd like to remember in the app's local database the name and the icon URL of each of the realms (organizations) the user is logged into. This means:

• we add the name and icon URL as fields on the Accounts table;
• we properly initialize them and keep them up to date.

The way we learn these values from the server is very similar to the Zulip feature level: they're in the responses from getServerSettings and registerQueue. I believe there's one added way they get updated: in RealmUpdateEvent. (Which I guess we haven't yet defined in our code! But see API doc.)