* Make it an error to supply empty API credentials, like this:
`https://danbooru.donmai.us/posts.json?login=&api_key=`. Some clients
did this for some reason.
* Make it so that the `login` and `api_key` params are only allowed as
URL params, not as POST or PUT body params. Allowing them as body
params could interfere with the `PUT /api_keys/:id` endpoint, which
takes an `api_key` param.
Remove the `CurrentUser.ip_addr` global variable and replace it with
`request.remote_ip`. Before we had to track the current user's IP in a
global variable so that when we edited a post for example, we could pass
down the user's IP to the model and save it in the post_versions table.
Now that we now longer save IPs in version tables, we don't need a global
variable to get access to the current user's IP outside of controllers.
