Avoid duplicate queries in auth

[ethantkoenig]

In the common case, two identical calls to GetUserByID(..) are made when SignedInUser(..) is called: one by SignedInID(..) (which SignedInUser(..) calls), and another by SignedInUser(..) itself.

I've refactored the code so that only at most one call to GetUserByID(..) is made. I also think it makes GetUserByID(..) a little more readable, since I was able to "get rid of" a long if statement.

[andreynering]

LGTM

[lunny]

LGTM