Replace 'organization' with British 'organisation' throughout application for consistency

:gb::gb::gb::gb::gb::gb::gb::gb::gb::gb::gb::gb::gb:
- all files and folders
- migrations on models
- controllers and views
- spec and feature tests
- rake tasks
- added a test for rake db:cat_org_import
- changed admin view of home page link 'Add Organisation' to differentiate from default footer link