Checking `plugin.enabled?` while initializing plugins causes issues in two ways:
- An application restart is required for changes to take effect. A load-balanced multi-server environment could behave very weirdly if containers restart at different times.
- In a multisite environment, it takes the `enabled?` setting from the default site. Changes on that site affect all other sites in the cluster.
Instead, `plugin.enabled?` should be checked at runtime, in the context of a request. This commit removes `plugin.enabled?` from many `instance.rb` methods.
I have added a working `plugin.enabled?` implementation for methods that actually affect security/functionality:
- `post_custom_fields_whitelist`
- `whitelist_staff_user_custom_field`
- `add_permitted_post_create_param`