Ignore repo syncs for projects without project id.

[asummers]

Fixes #1221

[joshsmith]

Just note to self that you're working this to:

• Add a regression test
• Fix the existing test failures

[asummers]

Things to consider moving forward:

There are "classes" of webhooks, some of which pertain to the repository and project and other that don't (installation, e.g.). As CC adds more webhook handlers for various reasons, the should_process? function will become increasingly further away from the handler logic, so this should eventually move down into the event handler logic, with the controller merely being a dispatcher.

[joshsmith]

With that change and a squash into a single commit we're good to merge this.