Remove Students and Forms menu items and set default landing page for Auditor accounts

Currently, since we treat Auditor accounts as a subset of an External Account the same way that Sending School Accounts are, Forms and Students show up in the navigation bar. No data can be seen due to security settings, but ideally, these should not be shown at all.

Additionally, the default home page for all users is the Upcoming Events page, but this is not relevant for Auditor accounts and should ideally be set to /Audit/Index