Follow-up of [1] PR above made several improvements to Messaging Menu, especially in mobile. By mistake, it removes the spacing between "All"|"Chats"|"Channels" and "New Message" in desktop. This commit re-adds this spacing with `div.flex-grow-1`. [1]: https://github.com/odoo/odoo/pull/140405 closes odoo/odoo#142186 Signed-off-by: Matthieu Stockbauer (tsm) <tsm@odoo.com>