a8n: External branch names should be more unique

Created by: ryanslade

Remote branch names are currently named campaign-id where id is and integer. There is a high chance of a conflict whenever we have a fresh db.

I propose we generate a unique id with each campaign that we'll use as part of external branch names.

A random sequence of 3 hex digits should be enough and won't make the branch name too noisy. We should also include a link in the PR back to the campaign in the sourcegraph instance that created it.