Bootstrap application and prepare toolchain

Summary:
This CLI application relies on keruald/commands and provides a
saas-phpbb command to run tasks related to phpBB or our SaaS
infrastructure.

Configure Toolchain:

• Arcanist: rSAASPHPBB @ DevCentral
• Composer: PSR-4 autoloader, BSD-2-Clause license
• PHPUnit: autoloader from Composer, tests folder
• PHP_CodeSniffer: Nasqueron codestyle

Ref T1382

Test Plan:

• bin/saas-phpbb.php
• vendor/bin/phan
• vendor/bin/phpcs
• arc lint
• arc which
• composer validate

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1382

Differential Revision: https://devcentral.nasqueron.org/D1570