The rake task was broken, because the addition of the
UploadSecurity check returned true/false instead of the
upload ID to determine which uploads to set secure.
Also it was rebaking the posts in the wrong place and
pretty inefficiently at that. Also it was rebaking before
the upload was being changed to secure in the DB.
This also updates the task to set the access_control_post_id
for all uploads. the first post the upload is linked to is used
for the access control. if the upload doesn't get changed to
secure this doesn't affect anything.
Added a spec for the rake task to cover common cases.
