Allow extensions to declare pages initialized with MandatoryDocumentInitializer as protected

Some JAR extensions contain MandatoryDocumentInitializer implementations that are responsible for initializing mandatory documents when the wiki is first accessed (i.e. at restart or subwiki access) or created.

These mandatory documents are not explicitly part of a xar package so they are not considered as being part of the extension that generated them. This means that these documents will not be protected against editing or deletion, among other things.

It would be interesting for a jar extension to be able to declare a list of document it produces and what should be the protection level for them, similar to the way we do it in a xar module int the packager plugin's configuration. They could be some special maven properties, similar to the ones used for extension pretty name or category, so that EM would pick them up when the extension is installed.