Clear Keychain after fresh install (iOS-315)

Files are removed when an app is deleted from device, but the Keychain is not cleared. So now if we find no cached account information upon launch, we remove any accounts remaining in Keychain and the app launches to the Welcome screen.

Fixes iOS-315

MastodonSDK/Sources/MastodonCore/AuthenticationServiceProvider.swift

@@ -166,14 +166,26 @@ public extension AuthenticationServiceProvider {
 
     @MainActor
     private func restoreFromKeychain() {
-        self.authentications = Self.keychain.allKeys().compactMap {
+        var keychainAuthentications: [MastodonAuthentication] = Self.keychain.allKeys().compactMap {
             guard
                 let encoded = Self.keychain[$0],
                 let data = Data(base64Encoded: encoded)
             else { return nil }
             return try? JSONDecoder().decode(MastodonAuthentication.self, from: data)
         }
         .sorted(by: { $0.activedAt > $1.activedAt })
+        let cachedAccounts = keychainAuthentications.compactMap {
+            $0.cachedAccount()
+        }
+        if cachedAccounts.count == 0 {
+            // Assume this is a fresh install.
+            // Clear the keychain of any accounts remaining from previous installs.
+            for authentication in keychainAuthentications {
+                try? delete(authentication: authentication)
+            }
+            keychainAuthentications = []
+        }
+        self.authentications = keychainAuthentications
     }
 
     func updateAccountCreatedAt(_ newCreatedAt: Date, forAuthentication outdated: MastodonAuthentication) {