Handle more gerrit events

[vthib]

Some use-cases that we want to develop:

• Notify people when they are set as reviewer of a commit (on first push, and explicitly). Good for small projects, not so much for bigger ones: must be activable per project.
• Notify people that reviewed a commit, when the owner replies to their reviews. Not easy to implement without useless notifications, one solution would be to replicate the behavior of emails (iirc):
  • Do not notify reviewers of a patch, unless they did at least one review
  • Notify those reviewers on every update of the commit (regardless of who made the update)
    Basically, when reviewing a commit, the reviewer indicates an interest on the commit, and receives all subsequents updates.