users: delete more data when user deactivates their account.

* Don't delete the user's favorites unless private favorites are enabled. The general rule is that
  public account activity is kept and private account activity is deleted.
* Delete the user's API keys, forum topics visits, private favgroups, downvotes, and upvotes (if
  privacy is enabled).
* Reset all of the user's account settings to default. This means custom CSS is deleted, where it
  wasn't before.
* Delete everything but the user's name and password asynchronously.
* Don't log the current user out if it's the owner deleting another user's account.
* Fix #5067 (Mod actions sometimes not created for user deletions) by wrapping the deletion process
  in a transaction.

app/controllers/users_controller.rb

@@ -131,7 +131,6 @@ class UsersController < ApplicationController
     user_deletion.delete!
 
     if user_deletion.errors.none?
-      session.delete(:user_id)
       flash[:notice] = "Your account has been deactivated"
       respond_with(user_deletion, location: posts_path)
     else