Make the security checklist consider HTTP 403 as a correct response.

Some web server configurations may return HTTP 403 when trying to access
certain filetypes. In this case, we should consider that an acceptable response
when testing whether dangerous files can be executed.

Testing done:
Jury rigged up something to throw a 403 HTTPError for a certain file extension.
Saw that before this change, I could reproduce the bug. After, it claims that
everything passes.

Fixes bug 3368.

Reviewed at https://reviews.reviewboard.org/r/5884/