Add "SkinCRC" column to Skins table for easing skin upgrade process

I propose to add "SkinCRC" column to Skins table. When user changes skin, then new crc will be calculated based on skin contents. This way, during upgrade process, we could determine fact, that user manually have edited skin. If skin wasn't manually edited, then we will overwrite it's contents based on "style_template.css" file.

Without CRC we have to manually upgrade skin after automatic site upgrade is made.