Completes the Greek translations in 19 files that were missing only a
few keys each, 45 keys in total. The files are now at full coverage.

Also fixes sylius.exchange_rate.ratio.greater_than, which had the value
hardcoded as 0 instead of using the {{ compared_value }} placeholder, so
the message was wrong whenever the comparison was against another value.
Its sibling less_than is added in this change and uses the placeholder,
so both now read the same way.