PERF: Improve theme stylesheet compilation performance (PR #12850)

When building the scss_load_paths, we were creating a full export of the theme (including uploads), and not cleaning it up. With many uploads, this can be extremely slow (because it downloads every upload from S3), and the lack of cleanup could cause a disk to fill up over time.

This commit updates the ZipExporter to provide a with_export_dir API, which takes care of cleanup. It also adds a kwarg which allows exporting only extra_scss fields. This should make things much faster for themes with many uploads.

GitHub

Removed /scss because the exporter will never put files in this location. They will always be placed under /stylesheets thanks to the canonical definition here:

Awesome, looks very nice!