add new notification_status_types static table

enums are hard to develop with - it is a slow and unchunkable change
to add a new type or remove an old type. We're slowly moving away from
using enums, in favour of static type tables, where the name is the
primary key.

This commit contains an alembic script that creates that table, and
creates a new nullable column on notifications and
notification_history that refer to it.

migrations/versions/0081_noti_status_as_enum.py

@@ -0,0 +1,33 @@
+"""empty message
+
+Revision ID: 0081_noti_status_as_enum
+Revises: 0080_fix_rate_start_date
+Create Date: 2017-05-02 14:50:04.070874
+
+"""
+
+# revision identifiers, used by Alembic.
+revision = '0081_noti_status_as_enum'
+down_revision = '0080_fix_rate_start_date'
+
+from alembic import op
+import sqlalchemy as sa
+
+
+def upgrade():
+    op.create_table('notification_status',
+        sa.Column('name', sa.String(), nullable=False),
+        sa.PrimaryKeyConstraint('name')
+    )
+
+    op.execute('ALTER TABLE notifications ADD COLUMN notification_status text')
+    op.execute('ALTER TABLE notification_history ADD COLUMN notification_status text')
+
+    op.create_index(op.f('ix_notifications_notification_status'), 'notifications', ['notification_status'])
+    op.create_index(op.f('ix_notification_history_notification_status'), 'notification_history', ['notification_status'])
+
+
+def downgrade():
+    op.execute('ALTER TABLE notifications DROP COLUMN notification_status')
+    op.execute('ALTER TABLE notification_history DROP COLUMN notification_status')
+    op.execute('DROP TABLE notification_status')