Switch LDAP to IDR

[orenyk]

Apparently we won't be able to use LDAP after this summer, so we need to start working out how to transition to IDR / the new People API. This will involved requesting access to the API, properly isolating the User model from the API (as well as UserImport), and making sure everything works. Ideally we'd build this in a way where we could use any arbitrary endpoint but authentication may be an issue. This is high priority!

[esoterik]

#1649 will isolate LDAP calls to make this switch easier!