Empty board and committee pages not consistent

Ghost User
2018-10-18T11:31:04+02:00

Going to the board page without any boards is not handled the same as going to the committee page without any committees.

### Current behaviour
If there are no boards, going to the board page results in a 404 page because of an error (`No Board matches the given query.`). Going to the committee page without any committees however shows a nice message "There are no committees!".

Looking at [the template for the board page](https://gitlab.science.ru.nl/thalia/concrexit/blob/master/website/activemembers/templates/activemembers/board_index.html#L57) it looks like a similar message should be shown:
```
{% empty %}
{% trans 'There are no boards!' %}
{% endfor %}
```

### Expected behaviour
If there are no boards, going to the board page results in `There are no boards!`.

### Steps to reproduce
1. Go to the boards page without any boards.