Reset unused cids, refactoring

app/src/main/java/eu/faircode/email/HtmlHelper.java

@@ -43,6 +43,7 @@ import java.io.FileOutputStream;
 import java.io.IOException;
 import java.io.InputStream;
 import java.net.URL;
+import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Date;
 import java.util.List;
@@ -288,4 +289,16 @@ public class HtmlHelper {
 
         return sb.toString();
     }
+
+    static List<String> getCids(String html) {
+        List<String> result = new ArrayList<>();
+
+        for (Element element : Jsoup.parse(html).select("img")) {
+            String src = element.attr("src");
+            if (src.startsWith("cid:"))
+                result.add("<" + src.substring(4) + ">");
+        }
+
+        return result;
+    }
 }