Notifications with the app closed, for every signed-in account

Second of three for background push across accounts (multi-account
spec, MA-8 part 2).

Turning on "Notify me even when inbuxa is closed" now registers this
browser in every signed-in account, each through its own session (the
route from the previous change), and renewal keeps them all current.
GET /api/auth/accounts says which mail account each session is, so the
subscription can name it. The remembered endpoint is kept per account,
and survives the clean-up that follows switching accounts.

The service worker is told who the other accounts are. A push for one
of them is titled with that account's address, shown even while a tab
is focused (the tab only shows the front account's mail), and its
Mark read and Archive act through that account's session. Clicking it
brings that account to the front and opens the message. While push is
on, the tab's own polling of other accounts stops notifying, so nothing
arrives twice.

Signing out of one account removes this device's subscription there
only; signing out of all, or turning push off, removes every one.

Also fixes where every background notification opened: the worker
linked to /mail/inbox/<thread>, and the route takes a mailbox id there,
so a click landed on the inbox list with "That folder no longer
exists". The worker now gets each account's inbox id and links to the
message.

Checked end to end in Chrome against a local server with two accounts:
both registered and verified, a message to the account not in front
showed a notification under its address, and clicking it switched
accounts and opened the message. No new strings. typecheck, tests
(web 1547, server 279) and build pass.

web/src/lib/sw/__tests__/swFacts.test.ts

@@ -44,6 +44,14 @@ describe("the worker's briefing", () => {
     expect(facts.archiveId).toBe("mb-archive");
   });
 
+  it("names the inbox a notification opens in", async () => {
+    // The route takes a mailbox id. The worker used to put the word `inbox`
+    // there, and every click landed on "That folder no longer exists".
+    const { store } = fakeCaches();
+    await publishWorkerFacts("a1", "mb-archive", "mb-inbox");
+    expect(written(store).inboxId).toBe("mb-inbox");
+  });
+
   it("carries the worker's text in the language the tab is in", async () => {
     // The worker has no catalog. Everything it will say has to be said here
     // first, or a German reader gets English buttons on their lock screen.