Set up webhook for Jenkins

[crawfxrd]

This requires someone with permission to add webhooks.

• https://github.com/system76/firmware-open/settings/hooks

It should trigger when a push is made to the repo. This can be limited to master; other branches can be manually run if required.

To avoid abuse, it should not trigger on PRs. I'm not sure if it even works if made from a fork, but, again, it can be manually run if required.

Still need to figure out what the hell needs to be done on Jenkins side.