Support `check` for invalid `template_name` string in `TemplateView` and other related types

[Sobolev Nikita]

python manage.py check helps us to find a lot of problems before actually running the project itself / tests for it.

I think that one thing that we can also check statically is template_name strings in different views that we have set-up in urls.
These templates can be found using regular Django machinery and help finging issues sooner.

Basically, I expect:

`python
from django.views.generic import TemplateView

class MyView(TemplateView):

template_name = 'my.html'

`

to raise an error during python manage.py check if my.html cannot be found with TEMPLATE loader settings. When MyView is present in urls.py as:

`python
urlpatterns = [

path('/whatever', MyView.as_view(), name='whatever'),

]
`

What do others think?