css: Move 10px navbar bottom padding above message header.

This is a more natural place for this extra padding than below the
navbar. Padding below navbar was primarily needed to hide the
message content above the sticky header while scrolling the message feed.
We are still able to do with this change without partially hiding the
loading message indicator.

Fixes #25159

web/src/message_list_view.js

@@ -1468,7 +1468,7 @@ export class MessageListView {
             // header has a box-shodow of `1px` at top but since it doesn't impact
             // `y` position of the header, we don't take it into account during calculations.
             const header_props = header.getBoundingClientRect();
-            // This value is dependent upon margin-bottom applied to recipient row.
+            // This value is dependent upon space between two `recipient_row` message groups.
             const margin_between_recipient_rows = 10;
             const sticky_or_about_to_be_sticky_header_position =
                 visible_top + header_props.height + margin_between_recipient_rows;