backport: split upgrade logic from ajax file

Conflicts:
	core/ajax/update.php
	lib/updater.php
backport: Updater: pass update url as argument to update check

Conflicts:
	lib/updater.php
fix typo
Stable5 adjustements (incl copy of lib/hooks/ from master) to make backport of #3963 work
Rerename \OC\Updater to OC_Updater, fixes broken admin page
Merge pull request #4047 from owncloud/backport_3617

Backport 3963