fix commit view JS features, reimplement folding (#9968)

* fix commit view JS features, reimplement folding

File content folding was not working so I reimplemented it in a saner
way. Then I noticed the issue was actually because of missing JS
libraries (seen on the console of every commit with error
'SimpleMDE is not defined').

Fixed the libraries. I think the reimplementation is worth to keep.

* add .closest polyfill

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

web_src/less/_repository.less

@@ -2524,3 +2524,11 @@ td.blob-excerpt {
 .title_wip_desc {
     margin-top: 1em;
 }
+
+.diff-file-box[data-folded="true"] .diff-file-body {
+    visibility: hidden;
+}
+
+.diff-file-box[data-folded="true"] .diff-file-header {
+    border-radius: 0.28571429rem !important;
+}