Deprecate GuildActions. Fix double case posting when muting a user for message spam. Update to new mute/case style when muting a user for "other" spam.

GuildActions turned out to be a fairly pointless abstraction in the end.
It didn't really solve the problems it was meant to solve (that is,
reduce code spaghetti by having all inter-plugin calls go through a
single service, and allow easier ways to replace core plugins with
alternatives that share the same interface) any better than simply
using `this.getPlugin()` when needed, and introduced extra complexity
and made static analysis messier.

src/utils.ts

@@ -504,7 +504,7 @@ export type UnknownUser = {
 };
 
 export const unknownUser: UnknownUser = {
-  id: "0",
+  id: null,
   username: "Unknown",
   discriminator: "0000",
 };