Handle multiple sites using the same admin email

[dantheta]

When the plugin registers a set of user credentials on blocked.org.uk, the resulting API secret is stored in wordpress database.

If a second site using the same admin email attempts to register with blocked, the user account creation will fail.

One solution would be to allow the API secret to be exported from the first site and entered manually on the second (an advanced setup option), so that both instances of the plugin are using the same credentials.

Alternatively, the plugin could generate a unique userid, and we could capture the admin email through some other means.

The UX will take a bit of thinking about.