Add the ability to invite admin users

[theosiemensrhodes]

When we deliver this product we will need a way for admins to invite other admins. They should be able to do this by email.

We should be able to use this better auth plugin in order to send these invitations. Once sent this should redirect to the sign up page where they collect the required info and on signup it should use the plugin accept invitation api call instead of the signUpEmail call.

We need some way to store which role the invite is for (instructor / admin). You should be able to work with this via the additionalFields field in the plugin options.