Move the events admin views to the admin model

One-sentence description

Move the events admin views to the admin model

Why?

It improves our codebase

Current implementation

We have a admin_views.py and admin.py. The admin views are routed through urls.py, not the admin.

Suggested implementation

Override the url config of the model admin used for events to add our custom views. Just like here: https://docs.djangoproject.com/en/2.1/ref/contrib/admin/#django.contrib.admin.ModelAdmin.get_urls