rubocop: fix various style issues.

config/application.rb

@@ -1,16 +1,16 @@
 require_relative 'boot'
 require "rails"
 require "active_record/railtie"
-#require "active_storage/engine"
+# require "active_storage/engine"
 require "action_controller/railtie"
 require "action_view/railtie"
 require "action_mailer/railtie"
 require "active_job/railtie"
-#require "action_cable/engine"
-#require "action_mailbox/engine"
-#require "action_text/engine"
+# require "action_cable/engine"
+# require "action_mailbox/engine"
+# require "action_text/engine"
 require "rails/test_unit/railtie"
-#require "sprockets/railtie"
+# require "sprockets/railtie"
 
 Bundler.require(*Rails.groups)
 
@@ -27,8 +27,8 @@ module Danbooru
     config.active_record.schema_format = :sql
     config.encoding = "utf-8"
     config.filter_parameters += [:password, :password_confirmation, :password_hash, :api_key]
-    #config.assets.enabled = true
-    #config.assets.version = '1.0'
+    # config.assets.enabled = true
+    # config.assets.version = '1.0'
     config.autoload_paths += %W(#{config.root}/app/presenters #{config.root}/app/logical #{config.root}/app/mailers)
     config.plugins = [:all]
     config.time_zone = 'Eastern Time (US & Canada)'
@@ -58,17 +58,17 @@ module Danbooru
       }
     end
 
-    if File.exists?("#{config.root}/REVISION")
+    if File.exist?("#{config.root}/REVISION")
       config.x.git_hash = File.read("#{config.root}/REVISION").strip
     elsif system("type git > /dev/null && git rev-parse --show-toplevel > /dev/null")
-      config.x.git_hash = %x(git rev-parse --short HEAD).strip
+      config.x.git_hash = `git rev-parse --short HEAD`.strip
     else
       config.x.git_hash = nil
     end
 
     config.after_initialize do
       Rails.application.routes.default_url_options = {
-        host: Danbooru.config.hostname,
+        host: Danbooru.config.hostname
       }
     end
   end