add configurable library refresh interval (#184)

* add configurable library refresh interval

* code cleanup

ErsatzTV.Infrastructure/Migrations/20210514110050_Add_LibraryRefreshInterval.cs

@@ -0,0 +1,15 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+namespace ErsatzTV.Infrastructure.Migrations
+{
+    public partial class Add_LibraryRefreshInterval : Migration
+    {
+        protected override void Up(MigrationBuilder migrationBuilder) =>
+            migrationBuilder.Sql(
+                "INSERT INTO ConfigElement (Key, Value) VALUES ('scanner.library_refresh_interval', '6')");
+
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+        }
+    }
+}