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/Groups.cs

@@ -227,7 +227,7 @@ namespace PluralKit.Bot
                 {
                     var eb = new EmbedBuilder()
                         .Title("Group icon")
-                        .Image(new(target.Icon));
+                        .Image(new(target.Icon.TryGetCleanCdnUrl()));
                     
                     if (target.System == ctx.System?.Id)
                     {