We have to make sure that plugins not override the context key 'extra_javasript' (with 'update'), so they have to use:
   "kwargs['context']['extra_javascript'].append('..')"
In utils/view.py we have to set an empty [] by default to append something later t it.
We use it in votecollector plugin, see https://github.com/OpenSlides/openslides-votecollector/blob/master/votecollector/views.py#L377