luci-app-nlbwmon: add proper file names for json and csv exports.

Currently, the json and csv exports are just named `data` which is not
intuitive. This adds proper file name extensions using the
Content-disposition HTTP header.

Suggested-by: Ameer Dawood <ameer1234567890@gmail.com>
[slightly reword commit message, squash commits]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>