Simplify the implementation for main wiki document initializers

The current AbstractMandatoryDocumentInitializer helper is focused on local documents, so any main-wiki-only document initializer needs to do unnecessarily complex work in each subclass.

The proposal is to support this in the helper base class and to expose a new method isMainWikiOnly that can be overridden to return true, signaling that the passed reference is a main wiki reference. Of course, if the passed reference already contains a wiki reference component, the method will be ignored and the reference will be used as initially passed.