Add validation rules to Infraction serializer

https://github.com/python-discord/site/issues/273

This commit adds validation rules to the Infraction serializer that
validate if a given infraction should be accepted based on its status
of being considered `active`. If the validation fails, the API will
reject the request and return a 400 status.

Specifically, this validator checks that:

- infractions that can never be active do not have `active=True` set;

- a user can never receive a second active infraction of the same type.

Tests have been added to `test_infractions.py` to ensure that the
validators work as expected.

This commit implements the first part of #273

0 files changed, 0 insertions, 0 deletions