Add logic to continuously groom regression label hygiene.

Following up on the proposal in https://gitlab.com/gitlab-org/gitlab-ce/issues/48518#proposal.

We want to add the following triage logic to gitlab-triage.

• If an issue only has the regression label, automatically add the ~bug label
• If an issue only has the regression:xx.x label, automatically add the regression label
• If an issue has the regression label but no regression:xx.x label, post a message to remind the author or EM to set a regression:xx.x label.

/cc @gl-quality