Extract configuration read/write methods out of BaseWorlkflow

Summary:
Create a new class for them, pass instance around as need.

This looks like it's mostly working, but I'd like to replace the various `new ArcanistConfigurationManager()`
calls with something more suitable.

And maybe get a better name for ArcanistConfigurationManager ("Configuration" is already taken).

Test Plan: arc unit --everything, and then some.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran, chad

Differential Revision: https://secure.phabricator.com/D7271