Remove fake img preview before loaded (#5854)

Remove fake img preview and collapse toggle before it is loaded,
only show img and toggle after it is fully loaded.
Fix markdown img size and add scroll down behaviour.

webapp/stores/scroll_store.jsx

@@ -0,0 +1,24 @@
+// Copyright (c) 2015 Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+import EventEmitter from 'events';
+
+const UPDATE_POST_SCROLL_EVENT = 'update_post_scroll';
+
+class ScrollStoreClass extends EventEmitter {
+    emitPostScroll() {
+        this.emit(UPDATE_POST_SCROLL_EVENT);
+    }
+
+    addPostScrollListener(callback) {
+        this.on(UPDATE_POST_SCROLL_EVENT, callback);
+    }
+
+    removePostScrollLisener(callback) {
+        this.removeListener(UPDATE_POST_SCROLL_EVENT, callback);
+    }
+}
+
+var ScrollStore = new ScrollStoreClass();
+export default ScrollStore;
+