Contract 6.20.0

The badge tools, after the Elixir Gym's first run on them (feedback #91-#94).

• badges_holders rows gain since: when the record first saw the badge at this level. observed_at is now the last profile poll that read the badge, as everywhere else. It used to be the stored change time, so 38 of 46 clan rows read as 17 days stale when every member had been re-read within a day. observations on both tools is the oldest and newest profile poll among the players considered.
• A versioned identifier says its version in its label. RoyalTournamentRank_v2 is Royal Tournament Rank (v2), and the single holder of the legacy badge no longer reads as the rarest badge in the game. badges_rarity notes a versioned pair when it lists both. 2v2 badges read 2v2 League Rank, not 2v 2.
• badges_holders takes a badge's label as well as its identifier. A label two badges share is refused with both identifiers. A miss is refused about the argument, with candidates, instead of 'no recorded player holds'.
• badges_holders serves holder_share (holders_total / players_considered) as its notes promised, and publishes its outputSchema.

Additive: since, holder_share; observed_at keeps its documented meaning.