Allow '\n' and other binary chars in dashboard CSV

By default, Text::CSV doesn't allow binary characters such as newline,
carriage return etc., in cells. This causes issues when exporting data
that contains these characters, such as problem reports with more than
one line.

This commit also explicitly tells Text::CSV to append a newline to each
row instead of joining the rows together with this char at the end.

Fixes mysociety/FixMyStreet-Commercial#494.