deal with old enchantment formats

[uncovery]

We list the complete former shopping history of users on the website. This includes items where the enchantment format is legacy such as serialized php arrays

a:4:{s:9:"sharpness";i:5;s:9:"knockback";i:2;s:10:"fireaspect";i:2;s:7:"looting";i:3;}

this format is invalid today. Either we

• remove all the past entries from a certain date on (a year or since a certain MC version) or
• we fix all the old entries or
• we simply remove the enchantments (but then the data would be "wrong", i.e. high prices for unenchanted items

[uncovery]

Done. Converted all old enchantments into new ones.