Show admin-wide alert if plugin is not ready to send!

[rajumsys]

We've observed a few confusions in WP Support forum that plugin cleans up settings upon uninstall. While we believe this is correct behaviour and plugin should continue doing so; plugin should alert when plugin is installed (activated) but not correctly configured.

https://wordpress.org/support/topic/re-enter-api-key-after-reactivate-plugin/

We can show a admin wide alert in such case, much like Akismet, IIRC.