add_index and remove_index method for migration on Django-1.11

[shimizukawa]

from: https://github.com/shimizukawa/django-redshift-backend/pull/24/commits/b551856bbe4528b191a6d21d3586a865db8c11af#diff-5af7c04b6b21eb52ef51f5ead82240ebR123

everything to do with a model's metaclass defining index_together.

add_index and remove_index have been added since django-1.11. I must investigate this

refs:

• https://github.com/django/django/commit/156e2d59cf92eb252c2f6ef9bb0b65f26c707de2
• https://github.com/django/django/commit/52442898e747e9d76bf6349938fa3dada6f0e887

[shimizukawa]

Redsihft doesn't support INDEX. https://docs.aws.amazon.com/en_us/redshift/latest/dg/c_unsupported-postgresql-features.html