Improve file header on mobile (#19945)

- File headers can become quite width, so ensure the file size is not
being wrapped into itself(width + padding-right) and allow the overflow
to be scrolled(overflow-x).

templates/repo/view_file.tmpl

@@ -1,6 +1,6 @@
 <div class="{{TabSizeClass .Editorconfig .FileName}} non-diff-file-content">
 	<h4 class="file-header ui top attached header df ac sb">
-		<div class="file-header-left df ac">
+		<div class="file-header-left df ac pr-4">
 			{{if .ReadmeInList}}
 				{{svg "octicon-book" 16 "mr-3"}}
 				<strong>{{.FileName}}</strong>