[FIX] mail: restore scroll in discuss app

min-height 0 is necessary for flex elements to properly shrink smaller than
their content. It was missing on this newly introduced parent.

Follow up on https://github.com/odoo/odoo/commit/cbe1bc809538f690115e1632448258663e1f52e4

task-2793274

closes odoo/odoo#86405

X-original-commit: 54df143917ed002190f5f628ce9bbc6840b4efd2
Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>
Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>

addons/mail/static/src/components/discuss_container/discuss_container.scss

@@ -0,0 +1,3 @@
+.o_DiscussContainer {
+    min-height: 0;
+}