Hid "Add another role" link on event edit page for people without assistant roles

Non TICs can only add run positions for their assistants, so if you hold a role that either is an assistant or cannot have any assistants, the "add another role" link is useless.
Fixed event role demotion vulnerability

Using a specially crafted request, a non-TIC run position with potential assistant positions could change the roles of other people with run positions for that event to the role of one of their assistants.