SCSS Processing Optimizations

[dansiegel]

Description

Currently SCSS Files are processed into CSS on every single build with a lot of noise. This results in processing that doesn't actually need to occur. This occurs in part due to all items with a None Include being evaluated as part of the process. This should be filtered to only files with the .scss extension. Ideally this should be further reduced to only processed on file update such as when a partial or a specific scss file is updated.

Generated CSS should also be moved to the Intermediate Output Directory to reduce the in project file noise.

[dansiegel]

@kzu I'm having some issues getting the SCSS files to be picked up by MSBuild. Not sure what we broke here... can you take a look at the sass-optimizations branch