users: add new owner level.

Add a new Owner user level for the site owner. Highly sensitive
operations like manually changing the passwords of other users will be
restricted to the site owner.

test/factories/user.rb

@@ -56,6 +56,11 @@ FactoryBot.define do
       can_approve_posts {true}
     end
 
+    factory(:owner_user) do
+      level { User::Levels::OWNER }
+      can_approve_posts {true}
+    end
+
     factory(:uploader) do
       created_at { 2.weeks.ago }
     end

test/mailers/previews/user_mailer_preview.rb

@@ -1,6 +1,6 @@
 class UserMailerPreview < ActionMailer::Preview
   def dmail_notice
-    dmail = User.admins.first.dmails.first
+    dmail = User.system.dmails.first
     UserMailer.dmail_notice(dmail)
   end
 

test/unit/user_test.rb

@@ -45,7 +45,14 @@ class UserTest < ActiveSupport::TestCase
     end
 
     should "normalize its level" do
+      user = FactoryBot.create(:user, :level => User::Levels::OWNER)
+      assert(user.is_owner?)
+      assert(user.is_admin?)
+      assert(user.is_moderator?)
+      assert(user.is_gold?)
+
       user = FactoryBot.create(:user, :level => User::Levels::ADMIN)
+      assert(!user.is_owner?)
       assert(user.is_moderator?)
       assert(user.is_gold?)