CSS reorganization and concatenation

- Reorganizes css and sass locations
- Adds a concatenation task to concatenate all the front end facing CSS files, except the IE8 (conditional) fix for Tiled Galleries
- Changes relative paths to resources used in CSS files in the concatenated file
- Adds a switch that serves the concatenated file instead of all the individual CSS files if the site is in production. Can be turned off with a filter.