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
Added composer.json
Merge remote-tracking branch 'origin/0.3'

Conflicts:
	code/SubsiteAdmin.php
	code/extensions/LeftAndMainSubsites.php
	code/extensions/SiteTreeSubsites.php
	code/model/Subsite.php
	composer.json
	javascript/SubsitesTreeDropdownField.js
	lang/en_US.php
	tests/SubsiteAdminTest.php
	tests/SubsiteTest.php
	tests/SubsiteTest.yml
	tests/SubsitesVirtualPageTest.php