Add horizontal scrollbar to tables

[simonw]

Currently you have to scroll the entire page sideways if a table is wide. Make the table overflow-x: auto instead.

[simonw]

This is particularly relevant to the datasette-cluster-map plugin - the map is much nicer to use if the table itself can be scrolled.

That plugin also makes this harder to build, because the plugin inserts the map as the direct predecessor of the <table> element and hence breaks if you try to wrap that in a <div>.

[simonw]

This may require updates to the column action menu JavaScript too, since it was not built with scrolling sideways in mind.

[simonw]

Dupe of #998.