Merge proposal webhooks don't have a scope for comments

[Giuseppe Barbieri]

The merge proposal webhooks don't really have a scope for comments, so any time something in the merge proposal changes, the webhook is triggered. Ideally we would have more fine grained scopes as it's currently available for bugs webhooks. This might require creating new scope versions for the merge proposal webhooks.

[Ines Almeida (ines-almeida)]

The current merge proposal webhooks are described in https://help.launchpad.net/API/Webhooks#merge-proposal-0.1

Indeed we don't show comments in the payload, but fire it when comments are added - this makes it hard to filter out comment triggers. Changing the scope of `merge-proposal-0.1` is not a good idea because some users of this webhook might already be expecting this behavior for their actions.

Possible solutions:
 - Add a "fields_updated" field to the payload that will help understand what changed that triggered the webhook
 - Create a new scope version for merge proposals (`merge-proposal-0.2`) that is more fine grained, and eventually one for comments specifically (`merge-proposal-comments-0.2`)