Fix message link when Discord does not provide `guild_id` (#1332)
Fix message link when Discord does not provide `guild_id` (#1332)

This commit creates a method `Message::link_ensured` that tries to search for the guild Id of a message if Discord does not provide it, as it is the case for messages received through the gateway. `Message::link` fails to generate a correct link when the message lacks the guild Id, as it will always assume the message comes from a DM.
Flat/serenity,acdenisSK/serenity