Improve gallery sizing logic

• improve gallery height calculation logic without tracking its container width
• fix a potential bug that doesn't show gallery attachments if the gallery (or timeline) was hidden at the mounting point
• set ideal flexible length on the gallery attachments based on their original aspect ratio (the original issue - !863 (comment 35649), you will see the ideal firefox version in all major browsers now)

src/components/attachment/attachment.js

@@ -10,7 +10,8 @@ const Attachment = {
     'statusId',
     'size',
     'allowPlay',
-    'setMedia'
+    'setMedia',
+    'naturalSizeLoad'
   ],
   data () {
     return {
@@ -88,6 +89,11 @@ const Attachment = {
       } else {
         this.showHidden = !this.showHidden
       }
+    },
+    onImageLoad (image) {
+      const width = image.naturalWidth
+      const height = image.naturalHeight
+      this.naturalSizeLoad && this.naturalSizeLoad({ width, height })
     }
   }
 }