Add an endpoint to cancel a job

If you schedule a job you might change your mind or circumstances might
change. So you need to be able to cancel it. This commit adds a `POST`
endpoint for individual jobs which sets their status to `cancelled`.

This also means adding a new status of `cancelled`, so there’s a
migration…

migrations/versions/0051_cancelled_job_status.py

@@ -0,0 +1,22 @@
+"""empty message
+
+Revision ID: 0051_cancelled_job_status
+Revises: 0050_index_for_stats
+Create Date: 2016-09-01 14:34:06.839381
+
+"""
+
+# revision identifiers, used by Alembic.
+revision = '0051_cancelled_job_status'
+down_revision = '0050_index_for_stats'
+
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects import postgresql
+
+def upgrade():
+    op.execute("INSERT INTO job_status VALUES ('cancelled')")
+
+def downgrade():
+    op.execute("UPDATE jobs SET job_status = 'finished' WHERE job_status = 'cancelled'")
+    op.execute("DELETE FROM job_status WHERE name = 'cancelled';")