API CHANGE Removed Subsite_Template subclass, as it unnecessarily duplicates the Subsite->duplicate() logic in Subsite_Template->createInstance(). It also arbitrarily limits duplication of subsites to templates.
SubsiteCopyPagesTask
NEW Copy page to different subsite, select MasterPageID

This is also the only UI-facing way to set a master page
after the initial copy action when creating a new subsite.
Shows "edit" link when master page is already set.
BUG Allow usage of SubsiteTreeDropdownField when SubsiteID is set in PHP, not through CopyContentFromID dropdown
NEW Access to non-public subsites for logged-in users

Also added caching subsite domain mapping
Fixed fixture class references
Fix test failures caused by subsite filtering on fixture retrieval