Review Bot has historically used the local directory it's being run from

as the location for the reviewbot-cookies.txt. That wasn't a very nice

experience, and required deployments to explicitly set $HOME to a

writable path. We had no safeguards in case the cookies couldn't be

written to, either.
This change makes the cookie storage path configurable, and gives it a

reasonable default (the XDG-defined user cache path).
Any failure to create the cookie directory or cookie file, or to write

to it, will trigger a fatal error at startup. This will contain explicit

information about what is wrong.
Existing reviewbot-cookies.txt files in the home directory will be

ignored, but Review Bot will just authenticate again the next time, so

it shouldn't impact any users.

Unit tests pass on Python 2.7 and 3.x.
Manually tested all success and error conditions with cookie storage

(on macOS).
Built the docs and checked for bad links, build errors, and spelling

errors.