require rubocop-rails in .rubocop.yml (#10974)

* Revert "Revert #10957 (rubocop-rails) which is incompatible with CodeClimate (#10965)"

This reverts commit 121d19d7.

* Disable Rails/HelperInstanceVariable

.rubocop.yml

+require:
+  - rubocop-rails
+
 AllCops:
   TargetRubyVersion: 2.3
   Exclude:
@@ -82,6 +85,9 @@ Rails/Exit:
     - 'lib/mastodon/*'
     - 'lib/cli.rb'
 
+Rails/HelperInstanceVariable:
+  Enabled: false
+
 Style/ClassAndModuleChildren:
   Enabled: false
 

Gemfile

@@ -129,6 +129,7 @@ group :development do
   gem 'letter_opener_web', '~> 1.3'
   gem 'memory_profiler'
   gem 'rubocop', '~> 0.71', require: false
+  gem 'rubocop-rails', '~> 2.0', require: false
   gem 'brakeman', '~> 4.5', require: false
   gem 'bundler-audit', '~> 0.6', require: false
 

Gemfile.lock

@@ -531,6 +531,9 @@ GEM
       rainbow (>= 2.2.2, < 4.0)
       ruby-progressbar (~> 1.7)
       unicode-display_width (>= 1.4.0, < 1.7)
+    rubocop-rails (2.0.0)
+      rack (>= 2.0)
+      rubocop (>= 0.70.0)
     ruby-progressbar (1.10.1)
     ruby-saml (1.9.0)
       nokogiri (>= 1.5.10)
@@ -735,6 +738,7 @@ DEPENDENCIES
   rspec-rails (~> 3.8)
   rspec-sidekiq (~> 3.0)
   rubocop (~> 0.71)
+  rubocop-rails (~> 2.0)
   sanitize (~> 5.0)
   sidekiq (~> 5.2)
   sidekiq-bulk (~> 0.2.0)