Reposition asset path build

[dv]

The standard config/initializers are only run after railtie#before_initialize, but we have to build the asset paths before railtie#after_initialize since at that point Sprockets has already frozen the asset paths.

This PR moves the asset path build to a point in between before_initialize and after_initialize, after the standard initializers are run but before Sprockets freezes the environment.

Fixes #345

[dv]

I spent a few hours looking on how to best test the initializer code, but apparently this is almost impossible - since we would have to completely reboot the Rails app, and I couldn't find anywhere how to do that in a test.

The compromise I've used here is to hardcode an initializer in the dummy app, which overrides a setting set in the dummy app's application.rb. The single test that checks the path now has the added task of checking if the addons option was set before Sprockets froze the world.

[rmosolgo]

Yeah, I remember trying to find some way to test it also, and basically giving up :P

But the way you tested it looks good to me. And it passes on all our Ruby & Rails versions :D

[rmosolgo]

Thanks! 🎊