Add favorites list plugin to application defaults

[shanbady]

What are the relevant tickets?

Closes https://github.com/mitodl/hq/issues/7544

Description (What does it do?)

This PR adds the FavoritesListPlugin plugin to application defaults. FavoritesListPlugin is responsible for automatically creating a placeholder favorites list for users upon signup.

How can this be tested?

NOTE: this already works locally. this fix is intended for deployed environments

1. checkout this branch
2. create a new user or login as existing
3. post-auth check to see that they have at least one favorites list via:

from django.contrib.auth import get_user_model
User = get_user_model()
user = User.objects.get(id=<your user id>)
assert user.user_lists.count() >= 1

[mbertrand]

👍