Add hooks into the platform to handle the workspaces feature when it is installed

The workspaces feature is being packaged as an extension but, in order to avoid it overriding some documents or templates, we must add some hooks into the platform to check if the extension is installed. If it is installed, then we can display the extension's UI elements in context.

Current requirements address:

• top menu template enhancements
• css for the top menu (common to both main wiki and workspaces)
• add/remove some sections in the administration section of a workspace
• add a section in the user profile for main wiki users
• hide the scope selector for user rights in a workspace and default to global

Please see http://markmail.org/thread/3cbms22ctbqi4yqp for more details on the topic.