Apply installation policy changes once each, per account

The last third of #207, and the only part that remembers anything.

An admin turns a setting on for people who are already here -- which a default
cannot do, since a default only seeds an account that has none -- and readers
may still turn it back off afterwards, which enforcement does not allow. The
difference between the two is entirely in the remembering.

Each change carries its own version, and an account stores the ones it has had
in its own settings file. Ids rather than a high-water mark, so a change dated
earlier than one already applied is not silently skipped -- the reporter's
analogy is a schema migration, and this is that shape.

Per account rather than per device, because ihasmail's settings are not
browser-local: they live in a file in the reader's own JMAP Files, with the
browser holding a cache. Signing in on a phone does not apply everything a
second time.

A change reaches somebody who had already decided otherwise. That is intended
and confirmed on the issue: the point is to reach everybody who is already
here. It is applied once, and their next decision sticks.

One `update` for however many are pending, since each would otherwise push a
settings file of its own. Enforced values still win, being applied after. A
change whose settings this build does not have at all is dropped rather than
recorded, or it would never run on the ihasmail that does have them.

The reader is told. A setting moving under somebody without a word is the part
of this worth being uneasy about, so the count is toasted with a way into
Settings.

README gains the Docker half the user asked for: a mounted policy file, the
same thing as environment variables for a deployment with no volume, a compose
fragment, and the fact that a policy is read once at startup so editing it
means a restart.

Closes #207.

web/src/App.tsx

@@ -9,7 +9,7 @@ import { useFiles } from "@/store/files";
 import { useSieve } from "@/store/sieve";
 import { push } from "@/jmap/push";
 import { client } from "@/jmap/client";
-import { ToastHost } from "@/ui/toast";
+import { ToastHost, toast } from "@/ui/toast";
 import { ConfirmHost } from "@/ui/dialog";
 import { Spinner } from "@/ui/misc";
 import { LoginPage } from "@/views/Login";
@@ -21,9 +21,9 @@ import { PAINTED_FROM_CACHE, useSettings, syncedPart } from "@/store/settings";
 import { armSettingsSync, loadRemoteSettings, queueSettingsPush, settingsAlreadyLoadedFor, settingsSyncAvailable } from "@/lib/settingsSync";
 import { loadSettingsPolicy } from "@/lib/settingsPolicy";
 import { listenForVerification, renewWebPush } from "@/lib/webpushEnable";
-import { useLanguageVersion, whenLanguageReady } from "@/lib/i18n";
+import { plural, t, useLanguageVersion, whenLanguageReady } from "@/lib/i18n";
 import { confirmLeaveUnsaved, hasUnsavedChanges } from "@/lib/unsavedChanges";
-import { BASE_PATH } from "@/lib/basePath";
+import { BASE_PATH, withBase } from "@/lib/basePath";
 
 const ContactsView = lazy(() => import("@/views/contacts/ContactsView").then((m) => ({ default: m.ContactsView })));
 const CalendarView = lazy(() => import("@/views/calendar/CalendarView").then((m) => ({ default: m.CalendarView })));
@@ -157,6 +157,19 @@ function AuthedApp() {
       // the installation's defaults are what it starts on rather than
       // ihasmail's. Issue #207.
       else useSettings.getState().seedFromPolicy();
+      /*
+       * After both, and for everybody: a change the installation wants applied
+       * once has to reach accounts that already exist, which is the whole of
+       * why it is not just a default. Each is remembered, so a reader who turns
+       * one back off keeps it off. Issue #207.
+       */
+      const applied = useSettings.getState().applyPolicyChanges();
+      if (applied.length) {
+        toast.show(plural(applied.length, {
+          one: "Your administrator changed {n} setting",
+          other: "Your administrator changed {n} settings",
+        }), { action: { label: t("Settings"), onClick: () => { window.location.href = withBase("/settings/general"); } } });
+      }
       // The catalogue for whatever language that turned out to be. Hydrating
       // asks for it; this is waiting for the answer.
       await whenLanguageReady();