FEATURE: disposable invite tokens

[Feature spec here](https://meta.discourse.org/t/invite-token-bulk-email-api-sample/17186/4?u=techapj).


You should put the `log_in` in a before block. In fact unless there's a reason you are inviting users using a different account than what logged in, why not make `let(:user) { log_in }`? That would save the repeating the log in every time