Log to application.log when an admin starts and stops impersonating a user

Closes gitlab-org/gitlab-ee#536

CHANGELOG

@@ -2,6 +2,7 @@ Please view this file on the master branch, on stable branches it's out of date.
 
 v 8.8.0 (unreleased)
   - Project#open_branches has been cleaned up and no longer loads entire records into memory.
+  - Log to application.log when an admin starts and stops impersonating a user
   - Make build status canceled if any of the jobs was canceled and none failed
   - Remove future dates from contribution calendar graph.
   - Support e-mail notifications for comments on project snippets

app/controllers/admin/impersonations_controller.rb

@@ -7,6 +7,8 @@ class Admin::ImpersonationsController < Admin::ApplicationController
 
     warden.set_user(impersonator, scope: :user)
 
+    Gitlab::AppLogger.info("User #{original_user.username} has stopped impersonating #{impersonator.username}")
+
     session[:impersonator_id] = nil
 
     redirect_to admin_user_path(original_user)

app/controllers/admin/users_controller.rb

@@ -41,6 +41,8 @@ class Admin::UsersController < Admin::ApplicationController
 
       warden.set_user(user, scope: :user)
 
+      Gitlab::AppLogger.info("User #{current_user.username} has started impersonating #{user.username}")
+
       flash[:alert] = "You are now impersonating #{user.username}"
 
       redirect_to root_path