Exclude certain users from leader board stats

[andreaskoepf]

• Add a new stats_enabled boolean column (default True) to the (User model) that indicates whether the user should be included in the leader board stats calculation or not (generate alembic migration script alembic revision --autogenerate -m "...").
• modify UserStatsRepository to exclude users that have stats_enabled == False
• allow updating stats_enabled via REST PUT to /api/v1/users/{user_id}, see update_user()

[akhil-datla]

@andreaskoepf I am interested in this task.

[andreaskoepf]

@akhil-datla please let us know if you have time to further work on #785. Otherwise we need to re-assign the task.