Make fileheader sticky in diffs (#14616)

* Make fileheader sticky #12552

* Remove sticky filenames when width is 480px or less

On mobile phone sticky filename is hidden due to the combination
of many possible widths and lengths.

* Fix text color for .markdown-info

* Fix visual of sticky diff box on 480px or less

- Hide arrow for select buttons.
- Fix changes, additions and deletions.
With flexbox they look very broken.
This commit hides some words to, so the result is:
"123 changed files  987 additions  456 deletions"
- center text in buttons

Co-authored-by: zeripath <art27@cantab.net>

templates/repo/diff/box.tmpl

@@ -79,7 +79,7 @@
 				</div>
 			{{else}}
 				<div class="diff-file-box diff-box file-content {{TabSizeClass $.Editorconfig $file.Name}}" id="diff-{{.Index}}">
-					<h4 class="diff-file-header ui top attached normal header df ac sb">
+					<h4 class="diff-file-header sticky-2nd-row ui top attached normal header df ac sb">
 						<div class="df ac">
 							{{$isImage := false}}
 							{{if $file.IsDeleted}}