Fix users cannot delete mumble account
Under certain circumstances (partial backup restore, table truncate, external management) the AuthServicesInfo and MumbleUser models can become unsynced, preventing a user from deleting or refreshing their Mumble account. This requires services admin to manually remove their mumble username in AuthServicesInfo which will then allow the user to create a new mumble account.
This PR adds a check for already deleted MumbleUser entities if the attempt to delete the users mumble account fails.