Add filetree on left of diff view (#21012)

This PR adds a filetree to the left side of the files/diff view.

Initially the filetree will not be shown and may be shown via a new
"Show file tree" button.

Showing and hiding is using the same icon as github. Folders are
collapsible. On small devices (max-width 991 PX) the file tree will be
hidden.

Close #18192

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/repo/commit_page.tmpl

@@ -1,7 +1,7 @@
 {{template "base/head" .}}
 <div class="page-content repository diff">
 	{{template "repo/header" .}}
-	<div class="ui container {{if .IsSplitStyle}}fluid padded{{end}}">
+	<div class="ui container fluid padded">
 		{{$class := ""}}
 		{{if .Commit.Signature}}
 			{{$class = (printf "%s%s" $class " isSigned")}}