Build request an API key feature into the plugin

[u451f]

The "user" registration API should work - so developers should be able to get API keys if we tell them about it - but it has no front end at present. We could consider building a "request an API key" feature into the Wordpress plugin's management page that would act as a front end for this API endpoint.

See https://wiki.openrightsgroup.org/wiki/Censorship_Monitoring_Project_API#POST_.2Fregister.2Fuser