MDL-67748 admin: Refactor tokens management as an external page
No need to have a dedicated admin_setting_managewebservicetokens admin setting subclass, and yet do not use almost any of its features. So the patch merges the list and the forms handling into a single external page. Similarly, it feels like overkill to have a renderer method for a simple confirmation widget - especially given that the URLs were hard-coded in it. So that one was dropped.