Convert `mummy/order` values to `long` type when parsing XHTML metadata.

Add logic to convert mummy/order values to long when parsing XHTML metadata.

Since we've been converting the mummy/order property (defined in Artifact) in HTML from a string to a long on the fly in AbstractPageMummifier.toLong(). We've had a TODO to switch to some general conversion system such as Ploop, and eventually we may want to still do that. Or not.

In we implemented special parsing for default-namespace properties in a certain form, but in that ticket we also anticipated eventually being able load ontology definitions for individual namespaces. For example we might have an ontology defined for the mummy/ namespace, indicating that mummy/order should be an urf-Number, so that it would automatically be converted to a long once, up-front when reading the document, and not on-the-fly as we use the value. There is already a placeholder for such logic in AbstractMummifier.parseMetadataPropertyValue().

So in the meantime we can add special-case logic for mummy/order to convert it to a long. The special logic will take the place of a facility for defining the mummy/ ontology in some declarative manner until we implement that.

This is relatively important to do so that the output site description can have the correct mummy/order type.

We may or may not keep the toLong() code and consider adding the general conversion facility.