[FIX] web: enterprise navbar toggle button -> a

This commit is a fix of the forward-port of an enterprise commit.
https://github.com/odoo/enterprise/commit/b0f309bc4ea79ff3ae2fe7d890dd60b0b414bd05

addons/web/static/src/less/navbar.less

@@ -6,7 +6,7 @@
     color: white;
     .box-shadow(0px 0px 8px @odoo-main-text-color);
 
-    > button {
+    > a, > button {
         float: left;
         width: @odoo-navbar-height - 2;
         height: @odoo-navbar-height - 2;
@@ -14,20 +14,23 @@
         padding: 0;
         line-height: @odoo-navbar-height - 2;
         background-color: transparent;
-        
+        text-align: center;
+        color: inherit;
+
         font-size: 18px;
         .user-select(none);
 
         &:hover, &:focus {
             background-color: @odoo-navbar-inverse-link-hover-bg;
+            color: inherit;
         }
         &:focus, &:active, &:focus:active {
             outline: none;
         }
+    }
 
-        &.o_menu_toggle {
-            margin-right: 5px;
-        }
+    > .o_menu_toggle {
+        margin-right: 5px;
     }
 
     > ul {