add allowedImageNames setting from ENV vars (#718)
add allowedImageNames setting from ENV vars (#718)

Uses `ALL_TEX_LIVE_DOCKER_IMAGES` (required) and
`ALL_TEX_LIVE_DOCKER_IMAGE_NAMES` (optional) to add the
`allowedImageNames` setting, allowing users to change a project's Tex
Live version.
sharelatex/sharelatex