Deploy to demo as Drone build promotion

[gabriel-v]

When https://github.com/drone/drone-ui/issues/171 will be solved we will get a button in the UI to promote builds. Until that is done, we could use the drone promote shell command, but I wouldn't get into that unless needed.

We should add a demo target and promote builds to that. The promote to demo pipeline should deploy the new code on liquiddemo.org.

We could proceed the same way for staging environments, just promote to staging!