Update to heroku-18 stack

RIGS/models.py

@@ -339,7 +339,7 @@ class Event(models.Model, RevisionMixin):
     auth_request_to = models.EmailField(null=True, blank=True)
 
     # Risk assessment info
-    risk_assessment_edit_url = models.CharField(max_length=255, blank=True, null=True)
+    risk_assessment_edit_url = models.CharField(verbose_name="risk assessment", max_length=255, blank=True, null=True)
 
     # Calculated values
     """

RIGS/versioning.py

@@ -27,6 +27,8 @@ class FieldComparison(object):
     def display_value(self, value):
         if isinstance(self.field, IntegerField) and len(self.field.choices) > 0:
             return [x[1] for x in self.field.choices if x[0] == value][0]
+        if self.field.name == "risk_assessment_edit_url":
+            return "completed" if value else ""
         return value
 
     @property

app.json

@@ -49,5 +49,6 @@
     {
       "url": "heroku/python"
     }
-  ]
+  ],
+  "stack": "heroku-18"
 }