Include used/remaining count in discount codes download file

[PhilDye]

As mentioned in issue #9, it should be possible to download a list of promo codes, with the count of number of times they've been used/remaining - this data is present in the underlying SQL table, but that's not available to a backoffice user.

[mattbrailsford]

@PhilDye I know this was a while ago, but there is a download button next to the promo codes. Is this the fix you were looking for? Or is there some lacking functionality in the download feature?

[PhilDye]

@mattbrailsford From memory, and having a quick look at an instance we still have, the feature was to include the uses/remaining count in the txt download (ie as csv) - it currently just includes the codes, with no counts

[mattbrailsford]

@PhilDye gotcha. I've updated he issue title to reflect this so it's clearer. Thank you.