Allow Configurable documents to request that they be copied on first load to ease upgradabulity.

Add a field to XWiki.ConfigurableClass which specifies the name of a document which the current document should be copied to in order to ease upgrading by shipping with a default configuration which creates a working configuration if one doesn't already exist.

The default configuration doc will be disregarded by ConfigurableClass and if the target doc name doesn't exist then the default document is copied to the target document and the target document has it's "copy to" field cleared to indicate that it is a working configuration.