Kill off old attachment management code

This commit removes all the code relating to the old way of managing
attachments. We no longer need to include the concern in our
controllers and we can kill off some of the macros that tested the
nested attachment behaviour (we still need one of them for now as it
is used for supporting pages and corporate information pages). We also
kill off the code that was used to manipulate the params based on any
bulk uploaded zip files.

Functional testing of attachments will now happen in
Admin::GenericEditionsController::AttachmentsWorkflowTest. There are
a couple of test cases still to be added there.
Add unimplemented test for display of attachments

This test replaces some of the tests that we've just removed...
Make BulkUpload::Attachments just BulkUpload

Turns out it makes a lot of sense to requisition the BulkUpload class
for our purposes here, and make it an ActiveRecord-like class that we
can use for the intermediary step when we process uploaded zip files.
WIP on bulk upload process

We now have a separate controller for the management of bulk uploads.
It will utilise our ActiveRecord-like object to eventually save a
collection of Attachments to an edition.
Rename method to explain what it does better
Reject non-zip files
Simplify BulkUpload::ZipFile

There's no need to inject the root dir as it never changes for a given
environment. Also moved the act of creating the main root directory
to the initializer so it is only called once, rather than in the
instance intialize method.