Replace 'GetChatLink()' with a pregenerated string

[sliekens]

/v2/items now has a chat_link field.

We already supported chat links in the form of Item.GetChatLink() : ItemChatLink. It would be appropriate to remove the GetChatLink() method and replace it with a ChatLink text property.

For /v1/item_details.json, the V1 item converters can be updated to generate the missing chat link.

We should probably do the same thing for recipes and POI chat links while we're at it.