Add IResettingEntitySystem for entity systems that do resetting cleanup (#2257)

* Add IResettingEntitySystem for entity systems that do resetting cleanup

* You got a license for that submodule update?

Content.Tests/Server/Preferences/ServerDbSqliteTests.cs

@@ -4,6 +4,7 @@ using System.Linq;
 using System.Threading.Tasks;
 using Content.Server.Database;
 using Content.Shared;
+using Content.Shared.GameTicking;
 using Content.Shared.Preferences;
 using Microsoft.Data.Sqlite;
 using Microsoft.EntityFrameworkCore;