issue 1858: handle, unhandle and remind an item, each with its own line (1858.2)

ADR 0517 §1, §2, §4, §5: an item is a task, a promise or the mail itself; each act and its action line are
kept in one transaction, a refused or failed act writes none, an undo lasts until the day ends and a
reminder shows the item again the next day (Europe/Brussels). The seat's serves list in mesh-mail's
manifest names the new verbs.

modules/mesh-mail/cmd/mesh-mail/manifest_test.go

@@ -148,7 +148,7 @@ func TestTheManifestSaysWhatTheCodeDoes(t *testing.T) {
 		{Name: "mail-store", Scope: "mesh", Serves: []string{"accounts", "search", "read", "send", "send-status"}},
 		{Name: "mail-webclient", Scope: "mesh", Serves: []string{"open", "status"}},
 		{Name: "mail-notifier", Scope: "node", Serves: []string{"status"}, Needs: []string{"node-notifier"}},
-		{Name: "mail-reading", Scope: "mesh", Serves: []string{"entries", "groups", "correct", "lessons", "undo", "status"}},
+		{Name: "mail-reading", Scope: "mesh", Serves: []string{"entries", "groups", "correct", "lessons", "undo", "status", "handle", "unhandle", "remind"}},
 	}
 	if !reflect.DeepEqual(m.Seats, wantSeats) {
 		t.Fatalf("seats %+v, want %+v", m.Seats, wantSeats)