Minor D8 changes

This is a collection issue for all the smaller D8 changes (not already listed explicitly in #2044421: [meta] Upgrade to Drupal 8) to which we'll have to adapt, like not using global $user.
I'm not entirely sure how to best go about this, but I guess I'll just collect all smaller changes I hear/read about here.

• global $user deprecated in favor of $request
• $_GET['q'] removed; must use current_path()
• user_access() got converted to a method on the user/accountInterface

Any additions are welcome!