Revive the Discord bot

[MartinKolarik]

• set it up locally, test all commands, fix any existing issues
• read Implement a Discord bot for usability globalping#397 and implement suggestions from there, especially around argument handling, output formatting (use all discord features that can improve the UX)
• add auth support and the related commands like in slack
• add limits command

Related: #52

Break this up into multiple PRs as you see fit.

[MartinKolarik]

• truncate embeds as needed, add the "full results available at..." message if truncation occurs:

• HTTP should default to HEAD, same as Slack.

[radulucut]

@MartinKolarik for the auth part, are we setting the token per server and user (for DMs) or just per user?

[MartinKolarik]

Per server and per user if run in DM. Let's make sure the server case is clear by printing a message when it's used.

[radulucut]

@MartinKolarik will discord have a different CLIENT_ID, CLIENT_SECRET and callback path (i.e '/discord/oauth/callback')?

[MartinKolarik]

Yes, all different.