Change default login redirect behavior for SAML SSO plugin

[SteelWagstaff]

When a user logins from the main Pressbooks landing page they are currently returned to that page as a logged in user. They can't do much except create a new book, however.

A better experience would be the following:

If the user does not have an existing book -- they are redirected to https://network.url/wp/wp-admin/user
If the user does have an existing book -- they are redirected to https://network.url/wp/wp-admin
See TK for current login-redirection behavior

Pseudo code with possible solution at https://github.com/pressbooks/pressbooks-oidc-sso/issues/28#issuecomment-707330562

We can also control what appears to the user on the user dashboard page by looking at https://github.com/pressbooks/pressbooks/blob/716d50e7d65c27c1e5acef9379fa19069855e039/inc/admin/dashboard/namespace.php#L50-L161