Default reviewers are added in bitbucket

Created by: sourcegraph-bot

User problem

In bitbucket, the open PR API call does not add default reviewers as in GitHub and GitLab. Adding reviewers manually to a batch change is impractical and requires a lot of back and forth between the Batch Changes GUI and bitbucket UI.

Proposed Solution

Default reviewers are automatically added when a PR is opened in bitbucket, just like in other codehosts. Note: Bitbucket provides a separate API for fetching the default reviewer for a repo.