Problem/Motivation As far as I can see, there is no method to get all flags for a particular user.

Steps to reproduce n/a

Proposed resolution Add a method to \Drupal\flag\FlagService - perhaps call it getFlagsByUser

Remaining tasks 1. add the method

1. use it in \Drupal\flag\FlagService::unflagAllByUser

User interface changes none

API changes Add new method on FlagService

Data model changes none