[CI] ESLint and Flow on each commit

Summary:

1. y eslint from repo root
2. ./node_modules/.bin/flow in each subdirectory

Note: we create empty secrets/db_config.json, facts/app_url.json, etc to appease flow in server directory

Test Plan: Successfully ran workflow in forked repo

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: palys-swm, Adrian, karol-bisztyga, benschac, boristopalov

Differential Revision: https://phabricator.ashoat.com/D2419