Use already defined constants instead of strings

Cherry-picked from GH-51.

src/main/java/de/thm/arsnova/ImageUtils.java

@@ -81,7 +81,7 @@ public class ImageUtils {
 		if (urlParts.length > 0) {
 			final String extension = urlParts[urlParts.length - 1];
 
-			return "data:image/" + extension + ";base64," + Base64.encodeBase64String(convertFileToByteArray(imageUrl));
+			return IMAGE_PREFIX_START + extension + IMAGE_PREFIX_MIDDLE + Base64.encodeBase64String(convertFileToByteArray(imageUrl));
 		}
 
 		return null;
@@ -186,9 +186,9 @@ public class ImageUtils {
 				g.dispose();
 
 				StringBuilder result = new StringBuilder();
-				result.append("data:image/");
+				result.append(IMAGE_PREFIX_START);
 				result.append(extension);
-				result.append(";base64,");
+				result.append(IMAGE_PREFIX_MIDDLE);
 
 				ByteArrayOutputStream output = new ByteArrayOutputStream();
 				ImageIO.write(newImage, extension, output);