fix bug that caused all plugins to be activated twice on startup

activate_plugins() is called from [PluginManager load_plugins](https://github.com/CarrotsAreMediocre/StarryPy/blob/16e002246a8daa4148a2f62f71d5aeeeb0105562/plugin_manager.py#L121), activating them in server.py was causing activate to be called on all plugins a second time when the server started,  causing db locks and possibly other nastiness.