Delay building a PR in case the developer makes several pushes in a row

[gurneyalex]

For the moment if a developer makes a series of commit + push we get 1 build per push. We should delay the build a little bit to save on ressources and only build the last push. Heuristic could be something like wait for more pushes on the branch for X minutes after the last push on that branch before building the head of the branch.

[sbidoul]

Alternatively, when deploying a new build, we could look at builds of the same PR that are initializing and undeploy them.
This is easier to implement and provides a better UX with faster initialization when the developer thinks before pushing :)
We could also run some analysis of the logs to see how prevalent this issue is.