Add a blank webapi/admin.py file.

This mirrors the recent addition of a blank models.py file. It helps
when switching back and forth between release-2.0.x and master branches.
Fix formatting errors in admin module according to PEP8

Fixed the following formatting errors:
admin/forms.py:808:13: E265 block comment should start with '# '
admin/forms.py:849:5: E265 block comment should start with '# '
admin/forms.py:850:5: E265 block comment should start with '# '
admin/forms.py:851:5: E265 block comment should start with '# '
admin/forms.py:852:5: E265 block comment should start with '# '
admin/forms.py:853:5: E265 block comment should start with '# '
admin/forms.py:900:5: E265 block comment should start with '# '
admin/validation.py:28:1: E302 expected 2 blank lines, found 1

Testing done:
Ran unit tests for admin module, and ran pep8 again.

Reviewed at https://reviews.reviewboard.org/r/6625/diff/#