Improve OverlayController

Updated: Comment #1

Problem/Motivation

OverlayController has fallen behind a bit on latest standards:

1. We can inject the token generator
2. We can extend ControllerBase to use $this->t() and $this->currentUser()
3. Also, #1978932: Convert overlay_ajax_render_region() to a Controller introduced some minor code-style problems

Proposed resolution

Fix the problems.

Related Issues

#1978932: Convert overlay_ajax_render_region() to a Controller
#1798296: Integrate CSRF link token directly into routing system
#1798296: Integrate CSRF link token directly into routing system
#2049159: Create a ControllerBase class to stop the boilerplate code madness
#2062151: Create a current user service to ensure that current account is always available