message_edit: Fix tab + enter broken for saving.

There is no `.message_edit_content` in `focus` when user does
`tab + enter` to save the message. We use the existing text
content edit object get identify the row being edited which is
more robust and efficient.

web/src/message_edit.ts

@@ -381,7 +381,7 @@ function handle_message_edit_enter(
 ): void {
     // Pressing Enter to save edits is coupled with Enter to send
     if (composebox_typeahead.should_enter_send(e)) {
-        const $row = $(".message_edit_content:focus").closest(".message_row");
+        const $row = $message_edit_content.closest(".message_row");
         const $message_edit_save_button = $row.find(".message_edit_save");
         if ($message_edit_save_button.prop("disabled")) {
             // In cases when the save button is disabled