Consider updating DEFAULT_ALLOWED_ATTRIBUTES

[xnx]

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).