[FIX] mail: missing spacing in Messaging Menu 'New Message'

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>

addons/mail/static/src/discuss/core/web/messaging_menu_patch.xml

@@ -2,6 +2,7 @@
 <templates xml:space="preserve">
     <t t-inherit="mail.MessagingMenu.content" t-inherit-mode="extension">
         <xpath expr="//div[hasclass('o-mail-MessagingMenu-header')]" position="inside">
+            <div class="flex-grow-1"/>
             <t t-if="ui.isSmall and (store.discuss.activeTab !== 'main' or !env.inDiscussApp)">
                 <div t-if="state.addingChat or state.addingChannel" class="w-100 p-1 bg-light" t-ref="selector">
                     <t t-if="state.addingChat" t-set="category" t-value="store.discuss.chats"/>