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

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

This reverts commit 121d19d7fa.

* Disable Rails/HelperInstanceVariable

.rubocop.yml

@@ -1,3 +1,6 @@
+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