Consider updating DEFAULT_ALLOWED_ATTRIBUTES

Currently this setting includes:

```
"tr": [
        "rowspan",
        "colspan",
    ],
```

but I don't think these are valid attributes for `<tr>`. However, it doesn't allow for `colspan` in `<th>` elements, where it would be very useful (I know it can be added to `"allowed_attributes"` in the `WAGTAILMARKDOWN` project setting, but it seems a harmless default).