FEATURE: Send notifications for time-based and At Desktop bookmark reminders (#9071)

* This PR implements the scheduling and notification system for bookmark reminders. Every 5 minutes a schedule runs to check any reminders that need to be sent before now, limited to **300** reminders at a time. Any leftover reminders will be sent in the next run. This is to avoid having to deal with fickle sidekiq and reminders in the far-flung future, which would necessitate having a background job anyway to clean up any missing `enqueue_at` reminders.

* If a reminder is sent its `reminder_at` time is cleared and the `reminder_last_sent_at` time is filled in. Notifications are only user-level notifications for now.

* All JavaScript and frontend code related to displaying the bookmark reminder notification is contained here. The reminder functionality is now re-enabled in the bookmark modal as well.

* This PR also implements the "Remind me next time I am at my desktop" bookmark reminder functionality. When the user is on a mobile device they are able to select this option. When they choose this option we set a key in Redis saying they have a pending at desktop reminder. The next time they change devices we check if the new device is desktop, and if it is we send reminders using a DistributedMutex. There is also a job to ensure consistency of these reminders in Redis (in case Redis drops the ball) and the at desktop reminders expire after 20 days.

* Also in this PR is a fix to delete all Bookmarks for a user via `UserDestroyer`

spec/lib/bookmark_reminder_notification_handler_spec.rb

@@ -0,0 +1,75 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe BookmarkReminderNotificationHandler do
+  subject { described_class }
+
+  fab!(:user) { Fabricate(:user) }
+
+  before do
+    SiteSetting.enable_bookmarks_with_reminders = true
+  end
+
+  context "when the user agent is for mobile" do
+    let(:user_agent) { "Mozilla/5.0 (iPhone; CPU iPhone OS 9_1 like Mac OS X) AppleWebKit/601.1.46 (KHTML, like Gecko) Version/9.0 Mobile/13B143 Safari/601.1" }
+    it "does not attempt to send any reminders" do
+      DistributedMutex.expects(:synchronize).never
+      send_reminder
+    end
+  end
+
+  context "when the user agent is for desktop" do
+    let(:user_agent) { "Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/60.0.3112.90 Safari/537.36" }
+    fab!(:reminder) do
+      Fabricate(
+        :bookmark,
+        user: user,
+        reminder_type: Bookmark.reminder_types[:at_desktop],
+        reminder_at: nil,
+        reminder_set_at: Time.zone.now
+      )
+    end
+
+    context "when there are pending bookmark at desktop reminders" do
+      before do
+        described_class.cache_pending_at_desktop_reminder(user)
+      end
+
+      it "deletes the key in redis" do
+        send_reminder
+        expect(described_class.user_has_pending_at_desktop_reminders?(user)).to eq(false)
+      end
+
+      it "sends a notification to the user and clears the reminder_at" do
+        send_reminder
+        expect(Notification.where(user: user, notification_type: Notification.types[:bookmark_reminder]).count).to eq(1)
+        expect(reminder.reload.reminder_type).to eq(nil)
+        expect(reminder.reload.reminder_last_sent_at).not_to eq(nil)
+        expect(reminder.reload.reminder_set_at).to eq(nil)
+      end
+    end
+
+    context "when there are no pending bookmark at desktop reminders" do
+      it "does nothing" do
+        DistributedMutex.expects(:synchronize).never
+        send_reminder
+      end
+    end
+
+    context "when enable bookmarks with reminders is disabled" do
+      before do
+        SiteSetting.enable_bookmarks_with_reminders = false
+      end
+
+      it "does nothing" do
+        BrowserDetection.expects(:device).never
+        send_reminder
+      end
+    end
+  end
+
+  def send_reminder
+    subject.send_at_desktop_reminder(user: user, request_user_agent: user_agent)
+  end
+end