support discarding tags, not just keys

[k-yle]

Currently, discarded.json only allows keys to be defined. This PR allows tags to be defined too.

For example, some values of attribution=* could be discardable, but not others.

{
  "created_by": true,
  "attribution": {
    "https://......import garbage.......": true
  }
}

it's a breaking change for any apps who use discarded.json.

---

this is required because openstreetmap/id-tagging-schema@a11968d is causing a weird inconsistency where iD is deleting some tags but not others (see this way's history)

[k-yle]

conflicts resolved