Often the actual release date does not exactly match with the first version date of for a given release.
Moreover the release date is simple to set : this is a one-time update that is not subject to change.
Currently update-product-data produces YAML with dates as string for unlisted releases. This may be an issue when running again the script after having copy-pasted the produced YAML. This fixes this issue.
Automatically remove empty releases or releases which are released in the future, regardless of the product.
Also refactored a bit releasedata.py to use better names and clarify types.
