[export] Mismatch between default footer filters and Auspice sidebar filters

[joverlee521]

Follow up to #1384 (comment)

Auspice has been updated to having sidebar filtering available for all node traits that are not continuous in nextstrain/auspice#1743 (not yet released). However in augur export v2, the default footer filters only use the colorings fields:

augur/augur/export_v2.py

Lines 539 to 542 in 4bf72f8

	# if not specified in the config, include all boolean and categorical colorbys
	potentials = {coloring["key"] for coloring in data_json['meta']["colorings"]
	if coloring["type"] != "continuous" and coloring["key"] != 'gt'}
	data_json['meta']['filters'] = list(potentials)

The default footer filters should probably be updated to include all node traits to match the Auspice sidebar filters.

[jameshadfield]

Now that Auspice will expose all available (non-continous) traits as sidebar-filtering options, I'd propose we drop any default "filter" values from Augur and make it explicitly opt-in. I don't think there's any need to deprecate it / remove it, as it's useful to some people, but we can make the "metadata columns" setting more prominant in the docs & recommend that as the method to include non-colouring data.