Added Service contact_link column

This is going to be used for for the document download citizen landing
page, a service will add a contact link
e.g. https://customerservicecontactnumber.uk/dwp/ which will allow the
user to contact the sending department if there is an error or any
issues with the download.

* Added the contact link to the model
* Added db migration script to add the column to the database

migrations/versions/0196_service_contact_link.py

@@ -0,0 +1,23 @@
+"""
+
+Revision ID: 0196_service_contact_link
+Revises: 0195_ft_notification_timestamps
+Create Date: 2018-05-31 15:01:32.977620
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+revision = '0196_service_contact_link'
+down_revision = '0195_ft_notification_timestamps'
+
+
+def upgrade():
+    op.add_column('services', sa.Column('contact_link', sa.String(length=255), nullable=True))
+    op.add_column('services_history', sa.Column('contact_link', sa.String(length=255), nullable=True))
+
+
+def downgrade():
+    op.drop_column('services_history', 'contact_link')
+    op.drop_column('services', 'contact_link')