Move mediaproxy URL rewriting to ProxyService

This shows full size avatars in API / cards.

Also, rewrite URLs currently stored with media.discordapp.net "back" to
cdn.discordapp.com before sending them to users.

PluralKit.Bot/Commands/SystemEdit.cs

@@ -203,7 +203,7 @@ namespace PluralKit.Bot
                 {
                     var eb = new EmbedBuilder()
                         .Title("System icon")
-                        .Image(new(ctx.System.AvatarUrl))
+                        .Image(new(ctx.System.AvatarUrl.TryGetCleanCdnUrl()))
                         .Description("To clear, use `pk;system icon clear`.");
                     await ctx.Reply(embed: eb.Build());
                 }