[FIX] mail: restores the documentViewer modal

Revision on
https://github.com/odoo/odoo/commit/34bbdd88c0bc354b76b254bb78c4eb786f3d08c0#diff-9da02f8f30ea349fbc4b9884c2b77b78

The document viewer did not work properly because, by default, dialogs are non-active,
so most interactions were not possible (just pressing ESC to close the dialog worked).

The correct fix consists of using OwlDialog for DocumentViewer,
but that's basically close to a re-writing, so in the meantime marking it as always active is fine.

closes odoo/odoo#43446

Task: 2177011
Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>

addons/mail/static/src/xml/thread.xml

@@ -87,7 +87,7 @@
         @param {mail.DocumentViewer} widget
     -->
     <t t-name="DocumentViewer">
-        <div class="modal o_modal_fullscreen" tabindex="-1" data-keyboard="false" role="dialog">
+        <div class="modal o_modal_fullscreen o_active_modal" tabindex="-1" data-keyboard="false" role="dialog">
             <t class="o_document_viewer_content_call" t-call="DocumentViewer.Content"/>
 
             <t t-if="widget.attachment.length !== 1">