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/MiscUtils.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Text.RegularExpressions;
 
 namespace PluralKit.Core
 {
@@ -20,5 +21,12 @@ namespace PluralKit.Core
 
             return true;
         }
+
+        // discord mediaproxy URLs used to be stored directly in the database, so now we cleanup image urls before using them outside of proxying
+        private static readonly Regex MediaProxyUrl = new Regex(@"^https?://media.discordapp.net/attachments/(\d{17,19})/(\d{17,19})/([^/\\&\?]+)\.(png|jpg|jpeg|webp)(\?.*)?$");
+        private static readonly string DiscordCdnReplacement = "https://cdn.discordapp.com/attachments/$1/$2/$3.$4";
+        public static string TryGetCleanCdnUrl(this string url) =>
+            MediaProxyUrl.Replace(url, DiscordCdnReplacement);
+
     }
 }