Start repo validation on activation

[roll]

• connects Trigger validation job after source has been added #210
  • start repo validation on activation
• small improvements for control flow in github integration

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For my piloting and fixing goodtables.io bugs work I've found almost blocking situation that you need to modify repository to get validation results (sometimes it's just not possible).

So for now I've added a trigger to start first validation on default branch straight after repo activation (also we could deactivate/activate repo to trigger validation at any time for any repo - priceless for testing).

It was a trivial change. Let's discuss later in #210 should we keep it or not. But I think for our non-technical users having immediate feedback from the system is essential. We just should add a note about it to Manage Sources page.

[amercader]

This looks good, but we definitely need a message that tells users that a job was triggered (here?). We should create an issue for that, which should be easy to implement.

[roll]

@amercader
I think we could cover it in more general e.g. here #213 adding help notes to different pages and something like how the service works link in header. For now both triggering on activation or triggering on push are not documented anywhere for new user. So both could be a surprise (or could not).