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/Utils/BulkImporter.cs

@@ -101,7 +101,7 @@ namespace PluralKit.Core
                     Description = patch.Description.Value,
 					Pronouns = patch.Pronouns.Value,
                     Color = patch.Color.Value,
-                    AvatarUrl = patch.AvatarUrl.Value,
+                    AvatarUrl = patch.AvatarUrl.Value?.TryGetCleanCdnUrl(),
                     KeepProxy = patch.KeepProxy.Value,
                     ProxyTags = patch.ProxyTags.Value,
                     Birthday = patch.Birthday.Value,