Use load instead of require_dependency in custom files

While using `require_dependency` to load original Consul Democracy code
from custom code works with the classic autoloader, this method was
never meant to be used this way. With zeitwerk, the code (apparently)
works in the test, development and production environments, but there's
one important gotcha: changing any `.rb` file in development will
require restarting the rails server since the application will crash
when reloading.

Quoting zeitwerk's author Xavier Noria, whom we've contacted while
looking for a solution:

> With the classic autoloader, when the Setting constant is autoloaded,
> the autoloader searched the autoload paths, found setting.rb in
> app/models/custom, and loaded it. With zeitwerk, the autoloader scans
> the folders in order and defines an autoload (Module#autoload) in
> Object so Ruby autoloads Setting with app/models/custom/settings.rb.
> Later, when app/models/setting.rb is found, it's ignored since there's
> already an autoload for Setting.
>
> That means the first file is managed by the autoloaders, while the
> second is not.
>
> So require_dependency worked, but it was pure luck, since the purpose
> of require_dependency is forcing the load of files managed by the
> autoloaders and, as we've seen, app/models/settings.rb isn't one of
> them.
>
> With your current pattern for custom files, the best solution is using
> Kernel#load.

So we're using `load` instead of `require_dependency`. Note that, with
`load`, we need to add the `.rb` extension to the required file, and we
no longer have to convert the Pathname to a string with `to_s`.

app/models/custom/setting.rb

@@ -1,4 +1,4 @@
-require_dependency Rails.root.join("app", "models", "setting").to_s
+load Rails.root.join("app", "models", "setting.rb")
 
 class Setting
   class << self