Override Block Explorer Links (#2000)

* Override Block Explorer Links

closes #1953

* load overrides after save as well

* fix js

BTCPayServer/Hosting/BlockExplorerLinkStartupTask.cs

@@ -0,0 +1,44 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading;
+using System.Threading.Tasks;
+using BTCPayServer.Services;
+
+namespace BTCPayServer.Hosting
+{
+    public class BlockExplorerLinkStartupTask : IStartupTask
+    {
+        private readonly SettingsRepository _settingsRepository;
+        private readonly BTCPayNetworkProvider _btcPayNetworkProvider;
+
+        public BlockExplorerLinkStartupTask(SettingsRepository settingsRepository,
+            BTCPayNetworkProvider btcPayNetworkProvider)
+        {
+            _settingsRepository = settingsRepository;
+            _btcPayNetworkProvider = btcPayNetworkProvider;
+        }
+
+        public async Task ExecuteAsync(CancellationToken cancellationToken = default)
+        {
+            var settings = await _settingsRepository.GetSettingAsync<PoliciesSettings>();
+            if (settings?.BlockExplorerLinks?.Any() is true)
+            {
+                SetLinkOnNetworks(settings.BlockExplorerLinks, _btcPayNetworkProvider);
+            }
+        }
+
+        public static void SetLinkOnNetworks(List<PoliciesSettings.BlockExplorerOverrideItem> links,
+            BTCPayNetworkProvider networkProvider)
+        {
+            var networks = networkProvider.GetAll();
+            foreach (var network in networks)
+            {
+                var overrideLink = links.SingleOrDefault(item =>
+                    item.CryptoCode.Equals(network.CryptoCode, StringComparison.InvariantCultureIgnoreCase));
+                network.BlockExplorerLink = overrideLink?.Link ?? network.BlockExplorerLinkDefault;
+
+            }
+        }
+    }
+}