Drop support for runserver_plus & django_extensions

[foarsitter]

django_extensions seems not actively maintained and we rely on it because of runserver_plus.

In my option the features runserver_plus brings aren't valuable enough to keep the dependency around. runserver doesn't have an interactive debugger but I can't remember when I used that feature for the last time.

[browniebroke]

That sad to read, I personally use a few of the commands it provides on a near daily basis: show_urls and shell_plus mainly. A few others that I use less often are delete_squashed_migrations and graph_models. It's true that the repo doesn't seem to have lots of activity, though.

[TGoddessana]

Recently, there was an issue where the Django extension did not provide the latest version of jupyter, forcing me to downgrade jupyter. Django is a mature framework and doesn't seem to be offering any groundbreaking API changes, but if it continues to not keep up with changes in Django or compatibility patches with other packages, it might be worth considering removing it from the templates.