add more friendly datetime validator to jsonschema

add `datetime` format (note, not the built-in `date-time`) to our json
schemas. this uses the iso8601 library to try and parse the string.

also, move `strict-rfc3339` and `rfc3987` (used by jsonschema to
validate `date-time` and `uri` formats respectively from test
requirements to regular requirements. if they're not installed,
validation silently succeeds, so validation wouldnt reject anything bad
on prod, only in unit tests.

app/schema_validation/__init__.py

@@ -54,6 +54,17 @@ def validate_schema_date_with_hour(instance):
     return True
 
 
+@format_checker.checks('datetime', raises=ValidationError)
+def validate_schema_datetime(instance):
+    if isinstance(instance, str):
+        try:
+            iso8601.parse_date(instance)
+        except ParseError:
+            raise ValidationError("datetime format is invalid. It must be a valid ISO8601 date time format, "
+                                  "https://en.wikipedia.org/wiki/ISO_8601")
+    return True
+
+
 def validate(json_to_validate, schema):
     validator = Draft7Validator(schema, format_checker=format_checker)
     errors = list(validator.iter_errors(json_to_validate))