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/utils/markdown.jsx

@@ -7,6 +7,8 @@ import * as SyntaxHighlighting from './syntax_highlighting.jsx';
 import marked from 'marked';
 import katex from 'katex';
 
+import ScrollStore from 'stores/scroll_store.jsx';
+
 function markdownImageLoaded(image) {
     if (image.hasAttribute('height') && image.attributes.height.value !== 'auto') {
         const maxHeight = parseInt(global.getComputedStyle(image).maxHeight, 10);
@@ -20,6 +22,7 @@ function markdownImageLoaded(image) {
     } else {
         image.style.height = 'auto';
     }
+    ScrollStore.emitPostScroll();
 }
 global.markdownImageLoaded = markdownImageLoaded;