Make the refactoring configuration more fine grained

Currently, the refactoring configuration is only configurable through the administration UI of each wiki and sub-wiki.

To ease its use we have to:

• use the configuration of the main-wiki as the default value for sub-wiki if they did not make an explicit choice
• use a configuration key from xwiki.properties as the default value if no explicit choice has been made

This is the same behaviour as the mail sender configuration.