public async Task <ActionResult> OnPostUnregister(int?id, [FromServices] UserManager <User> userManager)
{
User = await userManager.GetUserAsync(HttpContext.User);
CompetitionDto = _competitionService.GetCompetitions().Single(c => c.Id == id);
_participationService.removeParticipation(id, User.Id);
return(Redirect($"./Details?id=" + id));
}