Replace items instead of adding them (IOS-196)

Mastodon/Persistence/FileManager+SearchHistory.swift

@@ -28,6 +28,10 @@ extension FileManager {
 
         var searchItems = (try? searchItems(forUser: newSearchItem.userID)) ?? []
 
+        if let index = searchItems.firstIndex(of: newSearchItem) {
+            searchItems.remove(at: index)
+        }
+        
         searchItems.append(newSearchItem)
 
         let jsonEncoder = JSONEncoder()

Mastodon/Persistence/Model/SearchHistory.swift

@@ -5,11 +5,21 @@ import MastodonCore
 import MastodonSDK
 
 extension Persistence.SearchHistory {
-    struct Item: Codable {
+    struct Item: Codable, Hashable, Equatable {
         let updatedAt: Date
         let userID: Mastodon.Entity.Account.ID
 
         let account: Mastodon.Entity.Account?
         let hashtag: Mastodon.Entity.Tag?
+
+        func hash(into hasher: inout Hasher) {
+            hasher.combine(userID)
+            hasher.combine(account)
+            hasher.combine(hashtag)
+        }
+
+        public static func == (lhs: Persistence.SearchHistory.Item, rhs: Persistence.SearchHistory.Item) -> Bool {
+            return lhs.account == rhs.account && lhs.hashtag == rhs.hashtag
+        }
     }
 }