OZP-484: Set up compile_all_themes and watch_all_themes scripts to work from inside src.

1. Renamed each themes' sass folder to stylesheets for consistency and to enable container to more easily import dashboard's sass files, updated scripts to use it.
2. Changed container and dashboard's poms to execute the compile_all_themes script in the src directory again, instead of target.
3. Updated the sass config files for dashboard and container to import all the necessary files from inside src, removing configuration for compiling from inside target since it is no longer necessary.
4. Added gitignore files to theme css folders in container and removed gitkeep files.