Document: Static image items: Store an image URL, using the layout path.

Document: Static image items: Store an image URL, using the layout path.

        * src/main/java/org/glom/web/server/Utils.java: Add buildImageDataUrl().
        * src/main/java/org/glom/web/server/libglom/Document.java:
        Add a constructor that takes the documentID, for use in the
	LayoutItemImage URL.
	loadLayoutNode(): Pass a Path object to the helper methods, so we
	can use it to create a URL for LayoutItemImage items.
	This seems as good a way as any to specify a layout item in the document.
        * src/main/java/org/glom/web/server/ConfiguredDocumentSet.java:
	Pass the documentID to the constructor, for use in the LayoutItemImage
	URL.