[FIX] web_editor: set correct zindex on translation navbar

Commit c06f0f3a35 changed the base navbar zindex.
But the navbar of the translation mode still had old zindex values making the
navbar invisible, in fact under the base navbar.

task-1804662

addons/web_editor/static/src/less/web_editor.ui.less

@@ -4,7 +4,7 @@
     .o-position-absolute(0, 0, auto, 0);
     position: fixed;
     height: @odoo-navbar-height;
-    z-index: 1003;
+    z-index: @zindex-modal - 9;
 
     background-color: rgba(0,0,0,0);
     .o-transition(background-color, 400ms, @o-we-md-ease);
@@ -93,7 +93,7 @@ html[lang] > body.editor_enable [data-oe-translation-state] {
     .o-position-absolute(0, auto, 0, -@o-we-sidebar-width);
     position: fixed;
     width: @o-we-sidebar-width;
-    z-index: 1003;
+    z-index: @zindex-modal - 9;
 
     font-family: Roboto, @font-family-sans-serif;
     border-right: 1px solid @o-we-color-divider;