Read a card should dismiss all notifications

Previous code assumed that there was a single notification to dismiss
for that card, but in practice there can be zero or many. If it's zero,
we need to avoid the crash. If it's many, we should dismiss them all.

Also re-rendering the notification stack after dismissing enables us to
remove all the removed cards, as well as reveal any older ones that were
previously outside the visible page size.

This reverts commit 6135909b7e.

app/controllers/cards/readings_controller.rb

@@ -2,7 +2,7 @@ class Cards::ReadingsController < ApplicationController
   include CardScoped
 
   def create
-    @notification = Current.user.notifications.find_by(card: @card)
-    @notification.read
+    Current.user.notifications.unread.where(card: @card).read_all
+    @notifications = Current.user.notifications.unread.ordered.limit(20)
   end
 end

app/views/cards/readings/create.turbo_stream.erb

@@ -1 +1,3 @@
-<%= turbo_stream.remove @notification %>
+<%= turbo_stream.update "notifications" do %>
+  <%= render collection: @notifications, partial: "notifications/notification" %>
+<% end %>