reduce hard-coded CSS

[alexcjohnson]

Originally from https://community.plot.ly/t/datatable-formatting-issue-width/25133

Instead of things like

.dash-spreadsheet.dash-freeze-left {
    max-width: 500px;
}

What if we allowed

fixed_columns={'headers': True, 'data': 1, 'style': {'maxWidth': '800px'}}

where {'maxWidth': '500px'} is simply the default for (or merged into) fixed_columns.style?

Not sure what else fits that pattern - looks like fixed_rows has a similar issue, and potentially some other stuff in Table.less...