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
• home: Show org icon in app bar (Inbox, Channels, Direct messages) #1606, if this will include the app-bar-like part of the loading page we show while connecting

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.)