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/controllers/users/push_subscriptions_controller.rb

@@ -10,7 +10,7 @@ class Users::PushSubscriptionsController < ApplicationController
 
     # Existing endpoints must pass current validations too
     if subscription.valid?
-      head save_while_signed_in(subscription) ? :ok : :unauthorized
+      head register_while_signed_in(subscription) ? :ok : :unauthorized
     else
       head :unprocessable_entity
     end
@@ -27,12 +27,18 @@ class Users::PushSubscriptionsController < ApplicationController
     end
 
     # A password change ends every session and subscription. A request that authenticated just before it mustn't
-    # put one back, so the session is checked under the same write lock as the save. Validation, which looks up
-    # the endpoint's address, runs before taking the lock.
-    def save_while_signed_in(subscription)
+    # put one back, so the session is checked under the same write lock as the save, and the endpoint is looked up
+    # again there so two tabs posting at once register it once. Validation, which resolves the endpoint's address,
+    # runs before taking the lock.
+    def register_while_signed_in(subscription)
       Push::Subscription.transaction do
-        Session.exists?(Current.session.id) &&
-          (subscription.new_record? ? subscription.save!(validate: false) : subscription.touch)
+        if Session.exists?(Current.session.id)
+          if existing = @push_subscriptions.find_by(push_subscription_params)
+            existing.touch
+          else
+            subscription.save!(validate: false)
+          end
+        end
       end
     end