Fix N+1 for user on list in nav

[mikevallano]

app/views/layouts/_header.html.erb:47

We're querying for the list owner, but we shouldn't need it?

I changed set_list in the lists_controller to @list = current_user.all_lists.find_by(slug: params[:id]), but then tests were failing and I'm not really sure what the logic is supposed to be.

We can either update the logic based on how we're using the app, or if we still need that logic, do an includes to eliminate the N+1 queries, or could cache it since lists change infrequently.