make subnav sticky

we should review small devices

assets/scss/_tor.scss

@@ -259,6 +259,12 @@ mark, .mark {
     padding: 1rem;
 }
 
+.sticky{
+  position: sticky;
+  top: 110px;
+  z-index: 99;
+}
+
 /*
  * Sidebar
  */

assets/static/bootstrap.css

@@ -10217,6 +10217,12 @@ mark, .mark {
   padding: 1rem;
 }
 
+.sticky {
+  position: sticky;
+  top: 110px;
+  z-index: 99;
+}
+
 /*
  * Sidebar
  */

templates/pagenav.html

-<div class="container-fluid  border-bottom border-light">
+<div class="container-fluid border-bottom border-light bg-white sticky">
   <div class="row flex-xl-nowrap {{ bag('alternatives', this.alt, 'order') }}">
     <div class="row col-12 justify-content-center">
       <ul class="nav">