Add an getViewModelListBehaviorSubject that simplifies
how to get most model lists in the view
unified update, delete and create methods and removed redundant code from the repos
(where it was possible)
cleaned up the motion detail to not directly use the DataStore
Add information about the "main model" to all ViewModels, to call the
correct constructor in the BaseRepo
Used for internal apps as well as for plugins. The pluginpart is
currently missing, in fact that the main OpenSlides part is more
important. Apps can give their models and mainmenu entries.
Routes are not enabled, because the routes have to be static for webpack
to build the bundles. If we want to keep lazy loading, I see no
possibility to encapsulate the routes from the site-routing module.
