ServefileView should use FileResponse upon Django 2.2 upgrade

[vikoivun]

kerrokantasi/democracy/views/section.py

Line 555 in bf72d74

class ServeFileView(View, SingleObjectMixin):

should be changed to use FileResponse: https://docs.djangoproject.com/en/2.2/ref/request-response/#fileresponse-objects

Until rather recently FileResponse was slightly suboptimal and did not fill out the content-length field. In Django 2.1 it learned that too.