[#121] Remove django-cacheback

Remove code that used django-cacheback and its pip dependencies.

Remove unused imports in api/resources.py and reshuffle the importing
statements some.
Fix for #238 - Release notes for 2.1.5
[#212] Added akvo.rsr.models.Project.current_image_credit and associated South migration 0039.
[#212] Added migration 0039 file missing from previous commit.
Merge branch 'feature/image_credits' into develop
[#212] Added Photo credit field to Project page in Django Admin.
[#240] Remove dependencies used by django-cacheback

Cacheback had quite a few dependencies that aren't needed any more
[#224] Expose unpublished projects in project API resource

To be able to query the API when determining how to handle the
uploading of IATI formatted data we need access to unpublished projects
through the API.

Implementation works as follows:

Un-authenticated users have access to published projects only, as before.
An authenticated user also has access to unpublished projects that
would be accessible in the admin:
  - Superusers and Akvo staff groups members get access to all projects
  - Partner's users get access to their "own" unpublished projects

This is implemented by adding ProjectResource.get_object_list() and
Project.of_partner()
[#224] Refactor resources.py into its own module

Split resources.py into separate files under api/resources.
Some resources are grouped in one file based on sharing of the
underlying model or function of the resource.
[#244] Fixed bug in else: clause of akvo.rsr.admin.OrganisationAdmin.get_readonly_fields. Fix is a workaround and requires discussion.
Fix to Release notes - 190 problem
Test bug style fixed
Update RELEASE_NOTES.md

Minor typo fixes
[#244] Disabled help text hack in akvo.rsr.admin.OrganisationAdmin.get_readonly_fields override.
Merge branch 'release/2.1.5' of github.com:akvo/akvo-rsr into release/2.1.5
First commit of cordaid_org_importer.py script.
Merge branch 'feature/iati_update' of github.com:akvo/akvo-rsr into develop
[#250] Various improvements to cordaid_org_importer.py script. Now using .format() method and Python 3 print() calls.
[#250] Fix to Cordaid organisation creation script

Fix misspelling of variable name

Change the etree method for parsing the org XML
Merge branch 'release/2.1.5' into develop