Use django-attachments for user uploaded files. Fixes #160

- remove tcms/core/files.py
- remove now-unused home-grown attachment models
- remove related factories and update tests
- update templates
- update JavaScript files (remove unnecessary parts)
- update default permissions

As part of this change we no longer copy Plan and Case
attachments when cloning these objects.

NOTE: Since django-attachments introduces new permission objects
you will have to adjust default permissions for existing users.
In order for them to be able to upload/delete their own files they
need to have `attachments.add_attachment` and `atachments.delete_attachment`
permissions.

These same permissions are added by default to the 'Tester' group.
If you are running an existing installation registering a new user
with Kiwi TCMS will update the default permissions for this group!

Migrations for 'testcases':
    - Remove field attachment from testcaseattachment
    - Remove field case from testcaseattachment
    - Remove field case_run from testcaseattachment
    - Remove field attachment from testcase
    - Delete model TestCaseAttachment
Migrations for 'testplans':
    - Remove field attachment from testplanattachment
    - Remove field plan from testplanattachment
    - Remove field attachment from testplan
    - Delete model TestPlanAttachment
Migrations for 'management':
    - Remove field submitter from testattachment
    - Remove field attachment from testattachmentdata
    - Delete model TestAttachment
    - Delete model TestAttachmentData

docs/source/modules/tcms.core.rst

@@ -30,7 +30,6 @@ Submodules
    tcms.core.context_processors
    tcms.core.db
    tcms.core.exceptions
-   tcms.core.files
    tcms.core.managers
    tcms.core.middleware
    tcms.core.responses

requirements/base.txt

@@ -1,4 +1,5 @@
 Django==2.0
+django-attachments
 django-uuslug
 odfpy
 python-bugzilla