Theme module refactoring

Goals:

• use preprocessor + standard variables for branding customization
• remove the nuxeo styles prefixes (.galaxy**, ...)
• be able to use an application global css class (e.g. .nx)
• be able to use a page global css class (e.g. .nx-home, nx-admin, nx-search)
• simplify the resources definitions (css, js, fonts, icons, img)
• hot reload / live reload / websockets for styles
• remove theme & skin terminology: only resources. (we can keep flavor if needed)