Review Board's RB Gateway support requires POSTing to the /session API

(even though it doesn't actually post any form data -- it just passes an

Authorization header).
Also, the /repos/<repo>/path endpoint was mistakenly removed because it

does not return useful information. However, Review Board GETs this to

determine if authorization was successful, so I've added it back.

Ran unit tests.
Able to authorize in Review Board.