[Feature] Remove plugin cookie when deactivating/uninstalling

[MattRogowski]

When you uninstall, or maybe deactivate the plugin, it needs to remove the cookie that stores the password hash. Otherwise, when you reinstall and set a new password, it will try and use the hash of the old one as that's set in the cookie, and won't work if you set a different password when you reinstall.

[MattRogowski]

The cookie and key also needs to be cleared when the password is changed; that way they will have to enter the new password when they next upload and this will create a new key.