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.Core/Models/PKGroup.cs

@@ -29,6 +29,6 @@ namespace PluralKit.Core
             group.DescriptionPrivacy.Get(ctx, group.Description);
         
         public static string? IconFor(this PKGroup group, LookupContext ctx) =>
-            group.IconPrivacy.Get(ctx, group.Icon);
+            group.IconPrivacy.Get(ctx, group.Icon?.TryGetCleanCdnUrl());
     }
 }