Slack Integration with Read-only mode.

The new Bitbucket/Slack integration has a very rich feature set. Some of those features require privileges sufficient to make changes to the repository.

The features we want to actually use are much more restricted: essentially we just want to get notices in Slack when commits are made or Pull Requests sent; we emphatically don't want Slack to be able to make any changes at all.

What we'd like is either a way to grant the integration only read-only access to the repository regardless of the person configuring it.