Merge branch 'upgrade-doorkeeper' into 'master'

Upgrade Doorkeeper from 3.1.0 to 4.0.0.

Includes Rails 5 support and various bug fixes. Working toward #14286.

Changelog: https://github.com/doorkeeper-gem/doorkeeper/blob/master/NEWS.md#400

See merge request !5055

Gemfile

@@ -19,7 +19,7 @@ gem "pg", '~> 0.18.2', group: :postgres
 
 # Authentication libraries
 gem 'devise',                 '~> 4.0'
-gem 'doorkeeper',             '~> 3.1'
+gem 'doorkeeper',             '~> 4.0'
 gem 'omniauth',               '~> 1.3.1'
 gem 'omniauth-auth0',         '~> 1.4.1'
 gem 'omniauth-azure-oauth2',  '~> 0.0.6'

Gemfile.lock

@@ -171,8 +171,8 @@ GEM
     diff-lcs (1.2.5)
     diffy (3.0.7)
     docile (1.1.5)
-    doorkeeper (3.1.0)
-      railties (>= 3.2)
+    doorkeeper (4.0.0)
+      railties (>= 4.2)
     dropzonejs-rails (0.7.2)
       rails (> 3.1)
     email_reply_parser (0.5.8)
@@ -840,7 +840,7 @@ DEPENDENCIES
   devise (~> 4.0)
   devise-two-factor (~> 3.0.0)
   diffy (~> 3.0.3)
-  doorkeeper (~> 3.1)
+  doorkeeper (~> 4.0)
   dropzonejs-rails (~> 0.7.1)
   email_reply_parser (~> 0.5.8)
   email_spec (~> 1.6.0)