add broadcast_message to database

new broadcast_message table. contains a whole bunch of timestamps, a
couple of user ids for who created and who approved, some
personalisation and a template id/version. areas is a jsonb field - it
is expected that this will be an array, for example `["england",
"wales"]`

intended valid state transitions are as follows, from an initial status
of draft

* draft -> pending-approval, rejected
* pending-approval -> broadcasting, rejected, (draft maybe)
* broadcasting -> completed, cancelled

rejected, completed, cancelled and technical-failure are terminating
states.

also copy across the enum switching code from migration 0060 (adding
letter type).

app/models.py

@@ -2148,3 +2148,65 @@ class ServiceContactList(db.Model):
             "created_at": created_at_in_bst.strftime("%Y-%m-%d %H:%M:%S"),
         }
         return contact_list
+
+
+class BroadcastStatusType(db.Model):
+    __tablename__ = 'broadcast_status_type'
+    DRAFT = 'draft'
+    PENDING_APPROVAL = 'pending-approval'
+    REJECTED = 'rejected'
+    BROADCASTING = 'broadcasting'
+    COMPLETED = 'completed'
+    CANCELLED = 'cancelled'
+    TECHNICAL_FAILURE = 'technical-failure'
+
+    STATUSES = [DRAFT, PENDING_APPROVAL, REJECTED, BROADCASTING, COMPLETED, CANCELLED, TECHNICAL_FAILURE]
+
+    name = db.Column(db.String, primary_key=True)
+
+
+class BroadcastMessage(db.Model):
+    __tablename__ = 'broadcast_message'
+    __table_args__ = (
+        db.ForeignKeyConstraint(
+            ['template_id', 'template_version'],
+            ['templates_history.id', 'templates_history.version'],
+        ),
+        {}
+    )
+
+    id = db.Column(UUID(as_uuid=True), primary_key=True)
+
+    service_id = db.Column(UUID(as_uuid=True), db.ForeignKey('services.id'))
+    service = db.relationship('Service', backref='broadcast_messages')
+
+    template_id = db.Column(UUID(as_uuid=True), nullable=False)
+    template_version = db.Column(db.Integer, nullable=False)
+    template = db.relationship('TemplateHistory', backref='broadcast_messages')
+
+    _personalisation = db.Column(db.String, nullable=True)
+
+    status = db.Column(
+        db.String,
+        db.ForeignKey('broadcast_status_type.name'),
+        nullable=False,
+        default=BroadcastStatusType.DRAFT
+    )
+
+    # these times are related to the actual broadcast, rather than auditing purposes
+    starts_at = db.Column(db.DateTime, nullable=True)
+    finishes_at = db.Column(db.DateTime, nullable=True)  # isn't updated if user cancels
+
+    # these times correspond to when
+    created_at = db.Column(db.DateTime, nullable=False)
+    approved_at = db.Column(db.DateTime, nullable=True)
+    cancelled_at = db.Column(db.DateTime, nullable=True)
+    updated_at = db.Column(db.DateTime, nullable=True, onupdate=datetime.datetime.utcnow)
+
+    created_by_id = db.Column(UUID(as_uuid=True), db.ForeignKey('users.id'), nullable=False)
+    approved_by_id = db.Column(UUID(as_uuid=True), db.ForeignKey('users.id'), nullable=True)
+    cancelled_by_id = db.Column(UUID(as_uuid=True), db.ForeignKey('users.id'), nullable=True)
+
+    created_by = db.relationship('User', foreign_keys=[created_by_id])
+    approved_by = db.relationship('User', foreign_keys=[approved_by_id])
+    cancelled_by = db.relationship('User', foreign_keys=[cancelled_by_id])