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

horizon/forms/__init__.py:
Some entries were missing in __all__. They are added.

Change-Id: I33b504ef6c396f0675e8a340867e2ca59c77c684