Account-switcher modal

[gnprice]

This is a follow-up to:

• Show current organization's name and icon atop main menu #1037

Before this issue (and after #1037), when the user chooses the "Organizations" option from the main menu, we take them to a choose-account page with a list of accounts.

This issue is to instead show a modal menu with a list of accounts, like this:

This doesn't yet have a fully-updated design, so it will need that before it's ready for implementation.

This also has a post-launch issue as a prerequisite, in order to have all the needed data:

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

(Unlike #1037, this issue needs the realm name and icon even for accounts we don't currently have a live PerAccountStore for. That's why it needs that data to be in the database when #1037 doesn't.)

So this is a post-launch issue too.