Make github/gitlab integration use less invasive permissions

For many opensource projects it is important to maintain a tight control over who can have write access to their repositories. Sourcery.ai Github app is asking for wide permissions, including ability to push changes into the repositories. I’d like to see this tunable after the initial authorization.

In our experience Sourcery.ai pull request reviews are useful but we would like to avoid to see ability to push proposed changes into the PR tree under review. It would best be if such permissions were missing from the Sourcery.ai and such configuration was explicit in the app.sourcery.ai.