Apply Style/ClassVars rubocop rule

Class variables in Ruby are not the same as instance variables of a
class. They're particularly tricky when it comes to inheritance and
modules.

In the case of the Measurable module, for example, using a class
variable will make all classes including the Measurable module share
the same value. However, that's not what we want; we want the variable
to be different for every class. And that's accomplished using a class
instance variable.

Although in this case it would probably be better to remove the caching
variable. I don't think these methods are called more than once during a
request, and even if they did it's highly unlikely the would become a
bottleneck.

.rubocop.yml

@@ -376,6 +376,9 @@ Style/BlockDelimiters:
 Style/ClassCheck:
   Enabled: true
 
+Style/ClassVars:
+  Enabled: true
+
 Style/Not:
   Enabled: true
 

app/models/concerns/measurable.rb

@@ -3,11 +3,11 @@ module Measurable
 
   class_methods do
     def title_max_length
-      @@title_max_length ||= (columns.find { |c| c.name == "title" }.limit rescue nil) || 80
+      @title_max_length ||= (columns.find { |c| c.name == "title" }.limit rescue nil) || 80
     end
 
     def responsible_name_max_length
-      @@responsible_name_max_length ||= (columns.find { |c| c.name == "responsible_name" }.limit rescue nil) || 60
+      @responsible_name_max_length ||= (columns.find { |c| c.name == "responsible_name" }.limit rescue nil) || 60
     end
 
     def question_max_length

app/models/organization.rb

@@ -42,11 +42,11 @@ class Organization < ApplicationRecord
   end
 
   def self.name_max_length
-    @@name_max_length ||= columns.find { |c| c.name == "name" }.limit || 60
+    @name_max_length ||= columns.find { |c| c.name == "name" }.limit || 60
   end
 
   def self.responsible_name_max_length
-    @@responsible_name_max_length ||= columns.find { |c| c.name == "responsible_name" }.limit || 60
+    @responsible_name_max_length ||= columns.find { |c| c.name == "responsible_name" }.limit || 60
   end
 
   private

app/models/user.rb

@@ -313,7 +313,7 @@ class User < ApplicationRecord
   end
 
   def self.username_max_length
-    @@username_max_length ||= columns.find { |c| c.name == "username" }.limit || 60
+    @username_max_length ||= columns.find { |c| c.name == "username" }.limit || 60
   end
 
   def self.minimum_required_age