fixes #19528 - rewrite FiltersHelperOverrides to support autoload

Replaces class-level filter search overrides storage with a query
against Foreman::Plugin, allowing the class to be reloaded and tested
safely. Removes another 'registry' class.

fixes #19464 - rewrite Dashboard::Manager to support autoloading

Replaces class-level widget storage with a static list of built-in
widgets, then appends the registered plugin widgets to get the full
default list of widgets. As a result, the class can safely be reloaded...

fixes #19317, #10927 - move registries to autoload_once path

Partially reverts 70ce8fe to reinstate autoload_once_paths as the
preferred way to load registry-type objects with class variables
containing data set up during app initialisation (e.g. plugin
registrations, menu items.)...