Support for per-team consensus threshold

[RalfJung]

Currently rfcbot assumes all teams have the same threshold for when they reached consensus in an FCP: a strict majority, with at most 2 votes outstanding. In https://github.com/rust-lang/rust/issues/114986 the suggestion came up to make this per-team configurable instead, which seems like a useful feature for rfcbot.

The tricky part about this is that we'd have to first change the rules for multi-team FCPs: currently that's treated as if they all were one joint big team, but for per-team consensus rules we need to treat this as multiple per-team decisions, where consensus is reached when each team reached their own notion of consensus. This requires a global change to the consensus rules for all teams to transition multi-team FCPs from "one big team" to "multiple per-team decisions".