XARATINGS-4 : Allow admins to choose in which space the app will be active
- added a configuration class
- added XWiki.RatingsConfig as the global configuration file
- to configure the ratings on a specific space there has to be a XWiki.RatingsConfigClass object on the WebPreferences doc of that space
- the settings defined on a space have precedence over the ones from XWiki.RatingsConfig
- fallback to xwiki.cfg and xwiki.properties properties has been maintained (priority: space -> wiki -> xwiki.preferences -> xwiki.cfg)
- updated the UI Extension to be aware of the configuration objects