FIX: fallback to default push notification icon if none exists (#16961)

Due to some changes we started notifying via push notifications on other
families of notifications. There are a total of about 30 or so possible
notification you could get, some can be pushed.

This fallback means that if for any reason we are unable to find an icon
for a push notification we just fallback to the Discourse logo.

Also go with a simple reply icon for watching first post.

Note, that in production `image_url` can return an exception if an image is
missing. This is not the case in test / development.

app/services/push_notification_pusher.rb

@@ -5,7 +5,14 @@ class PushNotificationPusher
   CONNECTION_TIMEOUT_SECONDS = 5
 
   def self.push(user, payload)
+    message = nil
     I18n.with_locale(user.effective_locale) do
+      notification_icon_name = Notification.types[payload[:notification_type]]
+      if !File.exist?(File.expand_path("../../app/assets/images/push-notifications/#{notification_icon_name}.png", __dir__))
+        notification_icon_name = "discourse"
+      end
+      notification_icon = ActionController::Base.helpers.image_url("push-notifications/#{notification_icon_name}.png")
+
       message = {
         title: payload[:translated_title] || I18n.t(
           "discourse_push_notifications.popup.#{Notification.types[payload[:notification_type]]}",
@@ -15,7 +22,7 @@ class PushNotificationPusher
         ),
         body: payload[:excerpt],
         badge: get_badge,
-        icon: ActionController::Base.helpers.image_url("push-notifications/#{Notification.types[payload[:notification_type]]}.png"),
+        icon: notification_icon,
         tag: payload[:tag] || "#{Discourse.current_hostname}-#{payload[:topic_id]}",
         base_url: Discourse.base_url,
         url: payload[:post_url],
@@ -26,6 +33,8 @@ class PushNotificationPusher
         send_notification(user, subscription, message)
       end
     end
+
+    message
   end
 
   def self.subscriptions(user)

spec/services/push_notification_pusher_spec.rb

@@ -30,17 +30,27 @@ RSpec.describe PushNotificationPusher do
       PushSubscription.create!(user_id: user.id, data: data)
     end
 
-    def execute_push
+    def execute_push(notification_type: 1)
       PushNotificationPusher.push(user, {
         topic_title: 'Topic',
         username: 'system',
         excerpt: 'description',
         topic_id: 1,
         post_url: "https://example.com/t/1/2",
-        notification_type: 1
+        notification_type: notification_type
       })
     end
 
+    it "correctly guesses an image if missing" do
+      message = execute_push(notification_type: -1)
+      expect(message[:icon]).to eq("/assets/push-notifications/discourse.png")
+    end
+
+    it "correctly finds image if exists" do
+      message = execute_push(notification_type: 1)
+      expect(message[:icon]).to eq("/assets/push-notifications/mentioned.png")
+    end
+
     it "sends notification in user's locale" do
       SiteSetting.allow_user_locale = true
       user.update!(locale: 'pt_BR')