Add a hidden column to templates

Allows hiding templates from the templates list in the admin app
and related API responses.

This is used for 'internal' templates that we create for notifications
that wouldn't have a template otherwise (eg pre-compiled PDF letters)

app/models.py

@@ -664,6 +664,7 @@ class TemplateBase(db.Model):
     updated_at = db.Column(db.DateTime, onupdate=datetime.datetime.utcnow)
     content = db.Column(db.Text, nullable=False)
     archived = db.Column(db.Boolean, nullable=False, default=False)
+    hidden = db.Column(db.Boolean, nullable=False, default=False)
     subject = db.Column(db.Text)
 
     @declared_attr