Add !link command

[marcdif]

This command is the last step in the process to link a Minecraft account to a Discord account. This command should take one parameter - a numeric code the user gets from running /discord link on the Minecraft server.

This command should call https://internal-api.palace.network/discord/verify (not complete) with the necessary parameters. Regardless of the outcome, the user's message with the command should be deleted (to keep the channel clean).

If the code is valid, the user should be given the standard roles (as well as the display role according to their Palace Minecraft rank). This role change will hide the #link-account channel and display the rest of the channels, so no confirmation message is necessary (although it would be nice if Discord had ephemeral messages like slack).

If the code is invalid, the bot should send the user a DM informing them that the code was invalid. "The link code you provided was incorrect. Learn how to link your Minecraft account to the Palace Discord by visiting https://palnet.us/Discord".