Revert "Remove the Devise Async gem."

This reverts commit 1cc614f2.

It was causing the ActiveJob integration to fail, so unfortunately we'll have to add the gem again.

CHANGELOG

@@ -10,7 +10,6 @@ v 8.8.0 (unreleased)
   - Add 'l' shortcut to open Label dropdown on issuables and 'i' to create new issue on a project
   - Updated search UI
   - Display informative message when new milestone is created
-  - Replace Devise Async with Devise ActiveJob integration. !3902 (Connor Shea)
   - Allow "NEWS" and "CHANGES" as alternative names for CHANGELOG. !3768 (Connor Shea)
   - Added button to toggle whitespaces changes on diff view
   - Backport GitLab Enterprise support from EE

Gemfile

@@ -20,6 +20,7 @@ gem "pg", '~> 0.18.2', group: :postgres
 # Authentication libraries
 gem 'devise',                 '~> 3.5.4'
 gem 'doorkeeper',             '~> 3.1'
+gem 'devise-async',           '~> 0.9.0'
 gem 'omniauth',               '~> 1.3.1'
 gem 'omniauth-auth0',         '~> 1.4.1'
 gem 'omniauth-azure-oauth2',  '~> 0.0.6'

Gemfile.lock

@@ -164,6 +164,8 @@ GEM
       responders
       thread_safe (~> 0.1)
       warden (~> 1.2.3)
+    devise-async (0.9.0)
+      devise (~> 3.2)
     devise-two-factor (2.0.1)
       activesupport
       attr_encrypted (~> 1.3.2)
@@ -920,6 +922,7 @@ DEPENDENCIES
   database_cleaner (~> 1.4.0)
   default_value_for (~> 3.0.0)
   devise (~> 3.5.4)
+  devise-async (~> 0.9.0)
   devise-two-factor (~> 2.0.0)
   diffy (~> 3.0.3)
   doorkeeper (~> 3.1)

app/models/user.rb

@@ -91,7 +91,7 @@ class User < ActiveRecord::Base
   devise :two_factor_backupable, otp_number_of_backup_codes: 10
   serialize :otp_backup_codes, JSON
 
-  devise :lockable, :recoverable, :rememberable, :trackable,
+  devise :lockable, :async, :recoverable, :rememberable, :trackable,
     :validatable, :omniauthable, :confirmable, :registerable
 
   attr_accessor :force_random_password

config/initializers/devise_async.rb

+Devise::Async.backend = :sidekiq