Don't raise on invalid emails

[sentry-io]

Sentry Issue: STUDY-CREATOR-3Y

ValidationError: {'email': ['Enter a valid email address.']}
  File "graphql/execution/executor.py", line 452, in resolve_or_error
    return executor.execute(resolve_fn, source, info, **args)
  File "graphql/execution/executors/sync.py", line 16, in execute
    return fn(*args, **kwargs)
  File "creator/referral_tokens/mutations.py", line 102, in mutate
    referral_token.full_clean()
  File "django/db/models/base.py", line 1206, in full_clean
    raise ValidationError(errors)

An error occurred while resolving field Mutation.createReferralToken