Fix ActiveRecord deprecations from Rails 6.1

[texpert]

There are several deprecation warnings on CI on Rails 6.1. This PR is fixing the issues.

• In PostDefault: Fix "Deprecate .reorder(nil) with .first / .first! taking non-deterministic result" message - see https://github.com/rails/rails/pull/36889
• in CustomFieldGroup: Fix Deprecation warning in 6.1 if a default_scope contains a where.not - see https://github.com/rails/rails/issues/42759
• Remove deprecated associations from the custom_fields_read mixin, do some micro-optimizations of the code