Add new user permission for uploading executables

@heathd has added the new permission on preview and production where
appropriate. We shall use this to selectively permit certain users to
upload non-whitelisted files for attachments.
Remove pointless blank line
WIP - trying to skip whitelist verification

First pass at skipping the whitelisted file extension checks. This
attempts to get the uploader to skip verification if the owning
model says to. The problem is, the `model` is nil during the before
callback if the file is set using mass assignment....
Add first-pass bonkers controller code