Adds undocumented/experimental “master issue” feature.

Setting `config.masterIssue = true` will result in Renovate opening and maintaining an issue that contains a list of all PRs both pending and open, and allowing some control over them (e.g. bypass schedule, force retry, etc).

Setting `config.masterIssueApproval` in addition will mean that branches are not created automatically and instead await approval in that master issue.

Closes #2595