Clean up Sass compilation

The aim of this change is to separate compiling Sass files from copying them to the public directory, as they were combined in a way that made it hard to add more dirs. This release also fixes an issue with compiling Sass files under theme folders. The following (potentially) breaking changes have been introduced:

1. The :sass-dest key is no longer used. (It never was. That was an oversight with the original Sass integration.)
2. The :sass-src key now takes a vector of strings instead of just the one directory. This allows you to compile theme-specific scss files along with top level (under the templates folder) scss files.

If you are using Sass in your Cryogen templates, be sure to include these directories in your resources section as the compiled css will be outputted in the same directory and will need to be copied over to the public folder.