Bring back Mongoid official support (#5568)

Devise hasn't been tested with Mongoid since Rails version 5, only 4.x was still running those tests.

This enables the tests again on all currently supported Rails versions, with their respective mongoid supported versions. There were a couple of minor tweaks to make it happen, namely:

* The way we were dropping the session before doesn't work in later versions so I changed back to calling `purge!` which appears to work fine. We used to call `Mongoid.purge!` but that changed in #4686.
* Some of the configs in the Rails test app were setting Active Record values when outside of the AR ORM tests, updated those to make sure they are not set when running mongoid ORM tests.
* The validations added to the shared admin code in tests were only checking for Rails version 5.1, but we need to use the same check for AR 5.1 that is used in code, otherwise it will try to use methods not available in mongoid there.

test/orm/mongoid.rb

@@ -10,6 +10,6 @@ end
 
 class ActiveSupport::TestCase
   setup do
-    Mongoid.default_session.drop
+    Mongoid::Config.purge!
   end
 end

test/rails_app/config/application.rb

@@ -40,8 +40,10 @@ module RailsApp
       Devise::SessionsController.layout "application"
     end
 
-    if Devise::Test.rails70?
-      config.active_record.legacy_connection_handling = false
+    if DEVISE_ORM == :active_record
+      if Devise::Test.rails70?
+        config.active_record.legacy_connection_handling = false
+      end
     end
 
     if Devise::Test.rails70_and_up?