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

app/models.py

@@ -342,6 +342,7 @@ class Service(db.Model, Versioned):
     )
     crown = db.Column(db.Boolean, index=False, nullable=False, default=True)
     rate_limit = db.Column(db.Integer, index=False, nullable=False, default=3000)
+    contact_link = db.Column(db.String(255), nullable=True, unique=False)
 
     organisation = db.relationship(
         'Organisation',