scheduled_messages: Force save drafts of any length when scheduling.

Previously, if you wrote a two character message and attempted to
schedule it, there would be an assertion error on the draft_id
because we don't allow saving drafts with messages less than two
characters without force_save=true.

web/src/compose.ts

@@ -435,6 +435,7 @@ function schedule_message_to_custom_date(): void {
         no_notify: true,
         update_count: false,
         is_sending_saving: true,
+        force_save: true,
     });
     assert(draft_id !== undefined);