Related: #583

We had been using a single DB schema that didn't change until now.
This introduces migrations using `alembic`. To create a new migration
one can use:
```sh
alembic revision -m "My migration"
```
That should generate an empty migration file that needs to be hand-filled.
Specifically the `upgrade` method which will be the one executed when
running the migration.
```python
"""My migration

Revision ID: <some_hash>
Revises: <previous_hash>
Create Date: YYYY-MM-DD HH:MM:SS.XXXXXX
"""
from alembic import op
import sqlalchemy as sa

revision = '<some_hash>'
down_revision = '<previous_hash>'
branch_labels = None
depends_on = None

def upgrade():
    pass

def downgrade():
    pass
```