Stop duplicate push notifications and piling up subscriptions

Browsers subscribed to Email changes, so every read or move on any
client arrived as a push the worker could only show as "New mail". They
now subscribe to EmailDelivery, which changes only on delivery; Stalwart
sends a delivery to a subscription with an emailPush filter as an
EmailPush alone. The payload now names id and threadId, which Stalwart
sends only when asked, so notifications carry their actions and open the
message. The worker stays quiet while a focused window is open, and the
page leaves notifications to the worker where push is on.

Every renewal registered a new subscription, on the belief that a
repeated deviceClientId replaces the old one. Stalwart keeps both and
allows fifteen per account, which filled up. A browser now extends its
subscription, clears its own duplicates, replaces them only when its
endpoint changed, and on overQuota makes room among other browsers'
subscriptions. The server names its subscriptions by installation and
removes what its previous process registered, and extends rather than
re-creates.

Checked live on 0.16.22; the mock now keeps duplicates, enforces the
limit and accepts an expiry update.

Fixes #375.

web/public/sw.js

@@ -346,6 +346,14 @@ self.addEventListener("push", (event) => {
 
   const emails = (data && data["@type"] === "EmailPush" && Array.isArray(data.emails)) ? data.emails : [];
   event.waitUntil((async () => {
+    /*
+     * Someone reading the app already knows. A focused, visible window of this
+     * app gets its new mail from its own event stream, so a notification on
+     * top of it is a second telling of the same thing (#375). Chrome does not
+     * require one while the site is in the foreground.
+     */
+    const windows = await self.clients.matchAll({ type: "window" });
+    if (windows.some((w) => w.focused && w.visibilityState === "visible")) return;
     const facts = await readFacts();
     const strings = facts?.strings ?? { newMail: "New mail", newMessage: "New message", noSubject: "(no subject)" };
     /*
@@ -361,8 +369,10 @@ self.addEventListener("push", (event) => {
     if ("setAppBadge" in self.navigator) await self.navigator.setAppBadge().catch(() => {});
 
     if (!emails.length) {
-      // A StateChange, or a payload too large to carry the message. Say
-      // something true rather than inventing a sender.
+      // A delivery from a server that sends StateChange rather than EmailPush
+      // -- the subscription asks for `EmailDelivery` only, so it is new mail --
+      // or a payload too large to carry the message. Say something true
+      // rather than inventing a sender.
       await self.registration.showNotification(strings.newMail, {
         icon: `${BASE}/img/icon-192.png`, badge: `${BASE}/img/favicon-64.png`, tag: "ihasmail-mail", data: { url: `${BASE}/mail` },
       });
@@ -382,7 +392,8 @@ self.addEventListener("push", (event) => {
         // be drawn.
         actions: email.id ? actionsFor(facts) : [],
         data: {
-          url: email.id ? `${BASE}/mail/inbox/${email.id}` : `${BASE}/mail`,
+          // The route names a conversation, and `m` the message in it.
+          url: email.id && email.threadId ? `${BASE}/mail/inbox/${email.threadId}?m=${encodeURIComponent(email.id)}` : `${BASE}/mail`,
           id: email.id || null,
           title,
           accountId: facts?.accountId ?? null,