Add for attributes to all labels and ID's to all inputs

Lot of labels are missing `for=` attributes, which they really should have.
Merge pull request #1512 from jdevalk/patch-1

Add for attributes to all labels and ID's to all inputs
Fix undefined index on  when not using variable prices. #1478
Fix price option storage bug with PayPal buy now buttons. CLoses #1478
Merge branch 'master' of github.com:easydigitaldownloads/Easy-Digital-Downloads