Ensure Attachment upload fields are present

For various reasons, there are Attachment instances that are neither
a FileAttachment or an HtmlAttachment *. This ensures we always show
the file upload fields for non-HtmlAttachments.

* Firstly, the importer creates Attachments instead of
FileAttachments. This is easily rectified, but the main issue is that
STI and mass assignment don't play nicely, as the "type" attribute is
protected from mass-assignment in Rails < 4. This means that whenever
attachments are manipulated using mass-assignment (i.e. using the old
attachments interface on PolicyAdvisoryGroups and
CorporateInformationPages and), the type will get lost.
Ensure type is always set on response attachments
BulkUploader only creates FileAttachments
Be explicit about the type of attachments being built
Add note on abstract-ness of Attachment class

I tried to add code that prevents these from ever being saved, but
unfortunately that isn't very straighforward -The app currently uses
mass-assignment to enable the manipulation of attachments on
PolicyAdvisoryGroups and CorporateInformationPages, and
mass-assignment in Rails < 4 will not preserve the STI type attribute.

Either the admin interface for managing attachments on these models
should be updated to match the new one used on editions, or we should
upgrade to Rails 4, or ideally, both.
Get rid of Attachment factory

We shouldn't ever instantiate a base Attachment instance.

These factories are still fishy though - Firstly, they are created
without an attachable model, which shouldn't really be allowed.
They will also set the ordering an ordering based on a sequence,
unless it is explicitly provided. This means that the normal mechanism
for setting the ordering (based on the number of attachments already
attached to the attachable model) is bypassed, even if you build
the attachment and then assign it to an attachable model. We've got
away with this because the sequence will ensure uniqueness of ordering
and that they are incremental, but it does mean we are creating
weird test data in our tests.

I did make several attempts to clean this up, but there are a lot of
interdependencies in the factories for attachments and attachable
things like publications (which automatically add attachments) and
some of the traits like :with_file_attachment.
AttachmentBuilder builds FileAttachments
Delete redundant (skipped) tests

Attachments are no longer created with mass asssignment on editions
so these tests are redundant. They were being skipped anyway...
Drop redundant attachment test macro

The AdminEditionAttachableControllerTestHelpers module used to provide
a macro for testing the mass-assignment of attachments on editions.
This is no longer required as edition attachments now have their own
separate interface and are no longer mass-assigned. The tests that
are still relevant for corporate information pages have been moved
inline.
Manually merged branch 'fix-attachment-upload-form'

Tracker: https://www.pivotaltracker.com/story/show/61213224

Conflicts:
	test/factories/attachments.rb