Standardize less imports and image rule generation

* Combine mixins and variables into common file, which all styles include
* Always use common.less for styles and themes instead of using a variety of different includes
* Reorganize image styles to live within the images folder
* Generate image styles by defining mixins and then including a generic listing

Change-Id: Iccd3f827e2b7cb821ecd7c67c03061487c20fcdd