Add filter for discord invites.

[emmiegit]

Filter messages posted by non-moderators or whitelisted individuals, by checking for the existence of substrings like discord.gg, discord.me, and discordapp.com/invite.

An example regex that could suffice for this:

(https?:\/\/)*discord(app)?\.(gg|me|com)\/\w+

[kangalio]

I don't think this is a good idea, to be honest. There are many discord servers relevant to Rust that are brought up sometimes, for example The Coding Den or the Serenity Discord.