Fix migration and association for new members table

app/models/members/group_member.rb

app/models/members/project_member.rb

app/models/user.rb

app/services/system_hooks_service.rb

db/migrate/20140914113604_add_members_table.rb

db/migrate/20140914145549_migrate_to_new_members_model.rb

db/migrate/20140914145549_migrate_to_newmembers_model.rb

db/migrate/20140914173417_remove_old_member_tables.rb

db/schema.rb