In lab assistant sign-in, only include current options

[knrafto]

Reported by @kavigupta Cal-CS-61A-Staff/oh-queue#123

In the lab assistant sign-in, every possible combination of type/TA options is shown, an AI can choose, for example, Lab / Kavi during my office hours, even though I am not teaching a lab during that time.

It might be helpful if the Lab Assistant Manager only showed current options. An "other" field would also be required if, for example, someone was substituting for someone else.

[colinschoen]

Thanks, will fix.