Set filter according to PR/Change in URL

E.g. https://zuul-server/#333 will set filter to 333
Introduce related configuration option

And always pass the full change url in github status payloads
Allow the status_url to be used in a pipeline start-message.

Change-Id: I7b9eccb1d218d65d8c6b01e042e6cce4f8aec89a
4 files changed