Refactor how commitalert handles a new job

[tomrittervg]

In vendoring.py for vendoring tasks, we refactored things so that the first thing we ever do is create a job in the table. This means that if something goes wrong, we won't try continually processing the job.

We don't have the same design for commitalert tasks. (They're a lot rarer, so this hasn't ever come up.) We should probably change how commitalert works so it works similarly.