attribute-level imports are not checked by hacking module now.
most noqa is used to disable warnings on attribute-level imports.
This commit drops noqa for this purpose.
After this, there are 8 noqa under horizon/ :)
In addition to this, the following changes are made.
horizon/exceptions.py: The following imports are dropped.
They are not used anywhere.
from django.http import HttpRequest
from django.views.debug import CLEANSED_SUBSTITUTE
Some entries were missing in __all__. They are added.