Make use of Django fsm for invoice status flow

hypha/apply/dashboard/tests/test_views.py

hypha/apply/projects/filters.py

hypha/apply/projects/forms/payment.py

...pply/projects/migrations/0045_use_fsm_field_for_status.py

hypha/apply/projects/models/payment.py

hypha/apply/projects/tests/test_forms.py

hypha/apply/projects/tests/test_models.py

hypha/apply/projects/tests/test_templatetags.py

hypha/apply/projects/tests/test_views.py

hypha/apply/projects/views/payment.py

hypha/apply/users/models.py