add: Added .keep command which will add the current guild to the list of keptguilds. This is needed for the future database purge.

src/NadekoBot/Modules/Administration/DangerousCommands/_common/ICleanupService.cs

@@ -3,4 +3,5 @@
 public interface ICleanupService
 {
     Task<KeepResult?> DeleteMissingGuildDataAsync();
+    Task<bool> KeepGuild(ulong guildId);
 }