Improve members export: add user email field

I needed to have the email of the linked user of a UF member, so I added it to the SQL stored procedure. Maybe you find it interesting to have in upstream. I think it should appear in the export as it appears in the listing at the website.

PS: I don't know why the pull request shows two additional commits when they are already in upstream. Maybe I've messed up so much with my local branches or who knows :)