Remove unused import.
Disable not partial updates on attachment resources.

PUT request does not supports multipart request, but attached_file
is required. Because of this condition, put requests for attachments
resources are not possible, for it, them are disabled explicitly.
Check if project matches the content_object.project.
Remove deprecated attributes on attachments resources.
Remove unsed import on attachments permissions module.
Set attached_file field on attachments serializer required.
Remove usage of django-testclientextensions (backport of some commits from django1.7 branch)
Fix attachments texts.