Added alembic revisions for missing indices, unique constraints

I split the changes into two files since I noticed production had a
a unique key on subnet_id and address already on quark_ip_addresses,
but did not have any of the quark_mac_addresses changes.

Unfortunately, it's called `subnet_id` which is not possible with
alembic. Instead, I've called it `subnet_id_address` here.

Refers to commits in quark/db/models.py:
 - 375379d442656b97d78b77166ba43aa8048b3581
 - b4e0cde9cd8e149e1e5608c66be1f18b9d307917

Upgrading to latest SQLAlchemy allowed alembic to autogenerate the
relevant changes.
Merge pull request #146 from asadoughi/alembic_updates_6804

Added alembic revisions for missing indices, unique constraints