flow config webpack adjustements to avoid the "Required module not found" for png, css, svg etcc

.flowconfig

# ...

[options]
# webpack loaders
module.name_mapper='.*\.css$' -> '<PROJECT_ROOT>/flow/stub/css-modules.js'
module.name_mapper='.*\.\(svg\|png\|jpg\|gif\)$' -> '<PROJECT_ROOT>/flow/stub/url-loader.js'

flow__stub__css-modules.js

// @flow
type CSSModule = { [key: string]: string }
const emptyCSSModule: CSSModule = {}
export default emptyCSSModule

flow__stub__url-loader.js

// @flow
const s: string = ""
export default s