dispute-coordinator: disabling in participation (#2637)

Closes #2225.

- [x] tests
- [x] fix todos
- [x] fix duplicates
- [x] make the check part of `potential_spam` 
- [x] fix a bug with votes insertion
- [x] guide changes
- [x] docs

---------

Co-authored-by: Tsvetomir Dimitrov <tsvetomir@parity.io>

polkadot/primitives/src/v6/mod.rs

@@ -1340,12 +1340,8 @@ impl DisputeStatement {
 
 	/// Statement is backing statement.
 	pub fn is_backing(&self) -> bool {
-		match *self {
-			Self::Valid(ValidDisputeStatementKind::BackingSeconded(_)) |
-			Self::Valid(ValidDisputeStatementKind::BackingValid(_)) => true,
-			Self::Valid(ValidDisputeStatementKind::Explicit) |
-			Self::Valid(ValidDisputeStatementKind::ApprovalChecking) |
-			Self::Valid(ValidDisputeStatementKind::ApprovalCheckingMultipleCandidates(_)) |
+		match self {
+			Self::Valid(s) => s.is_backing(),
 			Self::Invalid(_) => false,
 		}
 	}
@@ -1374,6 +1370,19 @@ pub enum ValidDisputeStatementKind {
 	ApprovalCheckingMultipleCandidates(Vec<CandidateHash>),
 }
 
+impl ValidDisputeStatementKind {
+	/// Whether the statement is from the backing phase.
+	pub fn is_backing(&self) -> bool {
+		match self {
+			ValidDisputeStatementKind::BackingSeconded(_) |
+			ValidDisputeStatementKind::BackingValid(_) => true,
+			ValidDisputeStatementKind::Explicit |
+			ValidDisputeStatementKind::ApprovalChecking |
+			ValidDisputeStatementKind::ApprovalCheckingMultipleCandidates(_) => false,
+		}
+	}
+}
+
 /// Different kinds of statements of invalidity on a candidate.
 #[derive(Encode, Decode, Copy, Clone, PartialEq, RuntimeDebug, TypeInfo)]
 pub enum InvalidDisputeStatementKind {