fixes #22467 - dashboard: list hosts in build mode

Fixes #22779 - Calculate out of sync host via origin interval (#5221)

Fixes #21424 - use factory_bot_rails

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...