The Nijie login process works like this:
* First we submit our `email` and `password` to `https://nijie.info/login_int.php`.
* Then we save the NIJIEIEID session cookie from the response.
* We optionally retry if login failed. Nijie returns 429 errors with a
`Retry-After: 5` header if we send too many login requests. This can
happen during parallel testing.
* We cache the login cookies for only 1 hour so we don't have to worry
about them becoming invalid if we cache them too long.
Cookies and retrying errors on failure are handled transparently by Danbooru::Http.
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