CMS-4064 Remove Site Templates - enable special rendering of system.page-templates
Introduced client side caching of PageDescriptor, LayoutDescriptor and PartDescriptor to compensate against all the loading of such objects that seldom changes (changes only when module changes).
LiveFormPanel.initializePageFromDefault no longer needs to return a promise.
Made Content.metadata immutable and introduced method hasMetadata and getMetadata to simply controller programming.