Post the browser's push subscription from every page, and register it once

Only room pages posted the subscription again, so a browser that
changed its password and closed the profile page went without
notifications until it next opened a room. A small controller in the
layout does it now, still once per session, and the bell is back to how
it was.

Two tabs posting at once after a password change could both miss the
row and insert two, so the endpoint is looked up again under the same
write lock as the session check.

From #371.

(cherry picked from commit 3786ee5244)

app/views/layouts/application.html.erb

@@ -63,6 +63,11 @@
 
     <%= render "layouts/lightbox" %>
 
+    <% if Current.session %>
+      <%= tag.div hidden: true, data: { controller: "push-subscription-sync",
+            push_subscription_sync_url_value: user_push_subscriptions_path, push_subscription_sync_session_value: push_subscription_session_marker } %>
+    <% end %>
+
     <a href="https://once.com" id="app-logo" target="_blank" aria-label="Once software from 37signals home page">
       <%= image_tag "campfire-icon.png", alt: "Campfire logo", width: 256, height: 216 %>
     </a>

app/views/rooms/involvements/_bell.html.erb

@@ -1,7 +1,6 @@
 <span>
   <span class="button_to_change_notifying"
-      data-controller="notifications" data-notifications-subscriptions-url-value="<%= user_push_subscriptions_path %>"
-      data-notifications-session-value="<%= push_subscription_session_marker %>" data-notifications-attention-class="btn--pulsing">
+      data-controller="notifications" data-notifications-subscriptions-url-value="<%= user_push_subscriptions_path %>" data-notifications-attention-class="btn--pulsing">
     <%= turbo_frame_tag dom_id(room, :involvement), data: {
           controller: "turbo-frame", action: "notifications:ready@window->turbo-frame#load", turbo_frame_url_param: room_involvement_path(room) } do %>
       <button class="btn" data-action="click->notifications#attemptToSubscribe" data-notifications-target="bell">