Designate fields that submit multiple values correctly

Some fields can submit multiple values, i.e. use `[]` behind the name. These include `collection_check_boxes`, `select`, `collection_select`, and `grouped_collection_select`.

`strong_parameters` needs those to be designated specially, otherwise they won't be accepted:
```
add_signed_fields :user_ids => []
```
instead of
```
add_signed_fields :user_ids
```