Add a "setup" mode which guides new users through application configuration

Summary:
Alters the installation instructions to guide installers into a "setup" mode
which does config file sanity checking.

Test Plan:
Put myself in setup mode, simulated all the failures it detects, took myself out
of setup mode, Phabricator works OK.

Reviewed By: tuomaspelkonen
Reviewers: jungejason, tuomaspelkonen, aran
CC: aran, tuomaspelkonen, epriestley
Differential Revision: 230