Trigger validation job after source has been added

[roll]

Overview

For now adding source doesn't trigger any validation job:

The first job will be triggered the next time someone pushes to the GitHub repository.

As a {USER} I expect first validation triggered straight after the source has been added.

[roll]

@amercader no chance to have it in Gamma? For me as an user it's one of the most confusing thing - there is no autostart for newly added source and there is no button to start it from UI. So you need to do some dummy change in your repo to get first validation.

[amercader]

There should be some guidance on what happens after activating a source for sure. If we want to start a new job automatically we would need to know at least the default branch (normally master but not always). Also it would be good to ask the user explicitly so they know what it's about to happen.

I think it's important but the other issues for the milestone are a priority.