entry.attachments should include hidden items by default

[fluffy-critter]

Expected Behavior

If an entry has attachments with status: HIDDEN they should appear in the attachment view by default.

Current Behavior

There is no official support for showing status:HIDDEN attachments from a template; see #547

Possible Solution

Set the view defaults to include hidden=True in entry.attachments (and probably entry.attached as well).

Steps to Reproduce (for bugs)

Context

If you're attaching a thing to an entry, that entry should be able to see it.

[fluffy-critter]

Alternate idea: should view exclude attached items by default? Then there's no need to go through wonky hiding logic for them, and both this and #547 can go away. That might make more intuitive sense.