?_nofacets=1 query string argument for disabling facets and suggested facets

[simonw]

This is needed as an internal option for #1349. datasette-graphql can benefit from this too - maybe can even use it so that if you pass ?_shape=array it gets automatically added, fixing #263.

[simonw]

I think ?_facets_off=1 is a good design for this.

I considered ?_facet= with an empty string but that's a bit weird, and it's not clear what that would do if used like ?_facet=state&_facet=.

So it definitely needs to be a separate named option.

Actually, since I have ?_nocol= I'm going to call this ?_nofacets=1 instead as that's a little more consistent.

[simonw]

I renamed this to ?_nofacet=1 in #1353.