`?_trace=1` should only be available with a new `trace_debug` setting

[simonw]

Just like template debug mode is controlled by this off-by-default setting:

datasette/datasette/app.py

Lines 160 to 164 in 368aa5f

	Setting(
	"template_debug",
	False,
	"Allow display of template debug information with ?_context=1",
	),