Don't allow a feed to be in more than one folder for Reader API accounts

Account/Sources/Account/AccountBehaviors.swift

@@ -28,6 +28,11 @@ public enum AccountBehavior: Equatable {
 	*/
 	case disallowFeedInRootFolder
 	
+	/**
+	 Account doesn't support a feed being in more than one folder.
+	*/
+	case disallowFeedInMultipleFolders
+	
 	/**
 	Account doesn't support folders
 	*/

Account/Sources/Account/ReaderAPI/ReaderAPIAccountDelegate.swift

@@ -29,7 +29,7 @@ final class ReaderAPIAccountDelegate: AccountDelegate {
 	private var log = OSLog(subsystem: Bundle.main.bundleIdentifier!, category: "ReaderAPI")
 
 	var behaviors: AccountBehaviors {
-		var behaviors: AccountBehaviors = [.disallowOPMLImports]
+		var behaviors: AccountBehaviors = [.disallowOPMLImports, .disallowFeedInMultipleFolders]
 		if variant == .freshRSS {
 			behaviors.append(.disallowFeedInRootFolder)
 		}

Mac/MainWindow/Sidebar/SidebarOutlineDataSource.swift

@@ -644,6 +644,10 @@ private extension SidebarOutlineDataSource {
 			return true
 		}
 		
+		if violatesDisallowFeedInMultipleFolders(parentNode, draggedFeeds) {
+			return true
+		}
+		
 		return false
 	}
 	
@@ -677,6 +681,20 @@ private extension SidebarOutlineDataSource {
 		return false
 	}
 
+	func violatesDisallowFeedInMultipleFolders(_ parentNode: Node, _ draggedFeeds: Set<PasteboardWebFeed>) -> Bool {
+		guard let parentAccount = nodeAccount(parentNode), parentAccount.behaviors.contains(.disallowFeedInMultipleFolders) else {
+			return false
+		}
+		
+		for draggedFeed in draggedFeeds {
+			if parentAccount.hasWebFeed(withURL: draggedFeed.url) {
+				return true
+			}
+		}
+		
+		return false
+	}
+
 	func indexWhereDraggedFeedWouldAppear(_ parentNode: Node, _ draggedFeed: PasteboardWebFeed) -> Int {
 		let draggedFeedWrapper = PasteboardFeedObjectWrapper(pasteboardFeed: draggedFeed)
 		let draggedFeedNode = Node(representedObject: draggedFeedWrapper, parent: nil)