Validate notifcations.template_history foreign key constraints

Removes "NOT VALID" mark from the notifications -> template_history
foreign key constraint by validating existing records.

From postgres docs:

> This form validates a foreign key or check constraint that was
> previously created as NOT VALID, by scanning the table to ensure there
> are no rows for which the constraint is not satisfied. Nothing happens
> if the constraint is already marked valid.

> Validation can be a long process on larger tables. The value of
> separating validation from initial creation is that you can defer
> validation to less busy times, or can be used to give additional time
> to correct pre-existing errors while preventing new errors. Note also
> that validation on its own does not prevent normal write commands
> against the table while it runs.

> Validation acquires only a SHARE UPDATE EXCLUSIVE lock on the table
> being altered. If the constraint is a foreign key then a ROW SHARE
> lock is also required on the table referenced by the constraint.

migrations/versions/0142_validate_constraints.py

@@ -0,0 +1,21 @@
+"""
+
+Revision ID: 0142_validate_constraint
+Revises: 0141_remove_unused
+Create Date: 2017-11-15 14:39:13.657666
+
+"""
+from alembic import op
+from sqlalchemy.dialects import postgresql
+
+revision = '0142_validate_constraint'
+down_revision = '0141_remove_unused'
+
+
+def upgrade():
+    op.execute('ALTER TABLE notifications VALIDATE CONSTRAINT "notifications_templates_history_fkey"')
+    op.execute('ALTER TABLE notification_history VALIDATE CONSTRAINT "notification_history_templates_history_fkey"')
+
+
+def downgrade():
+    pass