Feature: Push notifications for Android (#5792)

* Feature: Push notifications for Android

Notification config for desktop and mobile are merged.

Desktop notifications stay as they are for desktop views.

If mobile mode, push notifications are enabled.

Added push notification subscriptions in their own table, rather than through
custom fields.

Notification banner prompts appear for both mobile and desktop when enabled.

app/services/push_notification_pusher.rb

@@ -0,0 +1,80 @@
+require_dependency 'webpush'
+
+class PushNotificationPusher
+  def self.push(user, payload)
+    subscriptions(user).each do |subscription|
+      subscription = JSON.parse(subscription.data)
+
+      message = {
+        title: I18n.t(
+          "discourse_push_notifications.popup.#{Notification.types[payload[:notification_type]]}",
+          site_title: SiteSetting.title,
+          topic: payload[:topic_title],
+          username: payload[:username]
+        ),
+        body: payload[:excerpt],
+        badge: get_badge,
+        icon: ActionController::Base.helpers.image_url("push-notifications/#{Notification.types[payload[:notification_type]]}.png"),
+        tag: "#{Discourse.current_hostname}-#{payload[:topic_id]}",
+        base_url: Discourse.base_url,
+        url: payload[:post_url],
+        hide_when_active: true
+      }
+      send_notification user, subscription, message
+    end
+  end
+
+  def self.subscriptions(user)
+    user.push_subscriptions
+  end
+
+  def self.clear_subscriptions(user)
+    user.push_subscriptions.clear
+  end
+
+  def self.subscribe(user, subscription, send_confirmation)
+    PushSubscription.create user: user, data: subscription.to_json
+    if send_confirmation == "true"
+      message = {
+        title: I18n.t("discourse_push_notifications.popup.confirm_title",
+                      site_title: SiteSetting.title),
+        body: I18n.t("discourse_push_notifications.popup.confirm_body"),
+        icon:  ActionController::Base.helpers.image_url("push-notifications/check.png"),
+        badge: get_badge,
+        tag: "#{Discourse.current_hostname}-subscription"
+      }
+
+      send_notification user, subscription, message
+    end
+  end
+
+  def self.unsubscribe(user, subscription)
+    PushSubscription.find_by(user: user, data: subscription.to_json)&.destroy
+  end
+
+  protected
+
+  def self.get_badge
+    return !SiteSetting.push_notifications_icon_url.blank? ?
+             SiteSetting.push_notifications_icon_url :
+             ActionController::Base.helpers.image_url("push-notifications/discourse.png")
+  end
+
+  def self.send_notification(user, subscription, message)
+    begin
+      response = Webpush.payload_send(
+        endpoint: subscription["endpoint"],
+        message: message.to_json,
+        p256dh: subscription.dig("keys", "p256dh"),
+        auth: subscription.dig("keys", "auth"),
+        vapid: {
+          subject: Discourse.base_url,
+          public_key: SiteSetting.vapid_public_key,
+          private_key: SiteSetting.vapid_private_key
+        }
+      )
+    rescue Webpush::InvalidSubscription => e
+      unsubscribe user, subscription
+    end
+  end
+end