Support API key reset request(s) from the mothership

To better protect users in a situation where the mothership has been compromised, let's support API key reset requests from the mothership.

Specifically, this comes in two parts:

* [x] WP Remote the app should be able to delete the API key stored in a given site (thus forcing a new API key to be entered). In situations where the user is applying an API key by a filter, the API request to delete an API key should also set an option that the filter checks (e.g. break the filter so the user has to perform some action).
* [x] WP Remote the plugin should periodically check WP Remote the app for a bat signal. If the bat signal is shown, then delete the API key and await further instructions.