Hiding features

[foolip]

#627 proposed a way to hide certain features, intended to map to caniuse's shown field. A lot of different names were proposed, among them legacy and archived.

I believe editorial judgment will be needed and that we need not try to compute this from other information.

(If trying to compute it, #623 based on a duration of time more than ~9 years, #623 would be a problem because the earliest possible Baseline low date is in July 2015.)

We need input from consumers, most notably caniuse, about what features they would like to hide when importing from web-features. cc @Fyrd