Graceful reload of Overleaf services

[HolgerHuo]

Currently, reloading overleaf must reload all services in a single docker container. It would be better to keep apis up while reloading frontends.

This feature might need to split multirepo into different containers. Better implementations welcomed.

[HolgerHuo]

To achieve following goals:

1. dynamically update build image configuration
2. update old compile images in previous objects
3. (maybe) admin interface to manage build images