Propagate digested paths for asset files from v3 to v2

[mekhovov]

Propagate 2 asset-url fixes from v3 to v2

• https://github.com/rails/sprockets-rails/pull/476 Process css files so that they get digested paths for asset files
• https://github.com/rails/sprockets-rails/pull/489 Add assets.resolve_assets_in_css_urls configuration option to allow disabling AssetUrlProcessor

We would like to use https://github.com/rails/dartsass-rails gem in our app. But we are not yet ready to upgrade sprockets to V3 and use manifest.js because of complex structure of our assets folders.

These 2 fixes in V2 allow us to use dartsass-rails gem with sprockets V2 and make asset urls work fine in dev. As well as pre-compile them for production