Remove one of our own jsonschema date-time formatters

We have three different ways of checking the formats of datetimes.
1. The built-in way that comes with the jsonschema package ("date-time")
2. A new way we added for broadcasts ("datetime") 61a5730596
3. An old way we defined in
   "/tests/app/public_contracts/schemas/v0/definitions.json"

In order to simplify things and make it clearer how datetimes are being
validated, this replaces the few places where we were using option 3 with option 1
instead. Option 3 was only being used to validate code that is no longer
used, the initial version of the API.

tests/app/public_contracts/schemas/v0/definitions.json

@@ -4,9 +4,5 @@
   "uuid": {
     "type": "string",
     "pattern": "^[a-fA-F0-9]{8}-[a-fA-F0-9]{4}-[a-fA-F0-9]{4}-[a-fA-F0-9]{4}-[a-fA-F0-9]{12}$"
-  },
-  "datetime": {
-    "type": "string",
-    "format": "date-time"
   }
 }

tests/app/public_contracts/schemas/v0/email_notification.json

@@ -14,19 +14,13 @@
       "type": "string",
       "enum": ["email"]
     },
-    "created_at": {"$ref": "definitions.json#/datetime"},
-    "sent_at": {"oneOf":[
-      {"$ref": "definitions.json#/datetime"},
-      {"type": "null"}
-    ]},
+    "created_at": {"type": "string", "format": "date-time"},
+    "sent_at": {"type": ["string", "null"], "format": "date-time"},
     "sent_by": {"oneOf":[
       {"type": "string"},
       {"type": "null"}
     ]},
-    "updated_at": {"oneOf":[
-      {"$ref": "definitions.json#/datetime"},
-      {"type": "null"}
-    ]},
+    "updated_at": {"type": ["string", "null"], "format": "date-time"},
     "status": {
       "type": "string",
       "enum": [

tests/app/public_contracts/schemas/v0/sms_notification.json

@@ -14,19 +14,13 @@
       "type": "string",
       "enum": ["sms"]
     },
-    "created_at": {"$ref": "definitions.json#/datetime"},
-    "sent_at": {"oneOf":[
-      {"$ref": "definitions.json#/datetime"},
-      {"type": "null"}
-    ]},
+    "created_at": {"type": "string", "format": "date-time"},
+    "sent_at": {"type": ["string", "null"], "format": "date-time"},
     "sent_by": {"oneOf":[
       {"type": "string"},
       {"type": "null"}
     ]},
-    "updated_at": {"oneOf":[
-      {"$ref": "definitions.json#/datetime"},
-      {"type": "null"}
-    ]},
+    "updated_at": {"type": ["string", "null"], "format": "date-time"},
     "status": {
       "type": "string",
       "enum": [