refactor the way imported_assets are added to the sitemap

- importing all images/fonts is moved to a separate method and run after_configuration. they get added to the imported asset list now
- assets handle their own resolution & directory manipulation using the passed in context from the app/sprockets_env
- assets will remove their “asset type” directory if it is in the logical path so they will be moved to the correct directory from config
- the output_path for an imported asset is now a bit more flexible to be a proc/labmda with or without arity