Reorder key hierarchy in theme.json

Each prefix can have its own set of settings.  At this time,
this is only used for configuring helpers depending on prefix.
In the future, this maybe use to configure menus/regions etc.
Configurable css classes in theme.json

- Configurations are read during application startup and sent to view as
  $themeSettings
- New helper method: LayoutHelper::cssClass() to retrieve configured values
CSS: Replace hardcoded container class
CSS: Replace hardcoded row class
CSS: Replace hardcoded span8 class
CSS: Replace hardcoded span4 class
CSS: Replace hardcoded span12 class
Adapt helpers to use css class configured in theme.json