Introduce pluggable email and image upload

src/main/java/com/openisle/service/ImageUploader.java

@@ -0,0 +1,14 @@
+package com.openisle.service;
+
+/**
+ * Abstract service for uploading images.
+ */
+public abstract class ImageUploader {
+    /**
+     * Upload an image and return its accessible URL.
+     * @param data image binary data
+     * @param filename name of the file
+     * @return accessible URL of the uploaded file
+     */
+    public abstract String upload(byte[] data, String filename);
+}