[Controller] Implement Status Conditions to reflect reconciliation progress

Summary

(Follow-up to !217 (merged))

The previous Operator set Status Conditions on the GitLab CR to indicate progress toward reconcilation (source code). This is a better place for Status updates than what we had previously (.status.{stage,phase}).

References

• https://github.com/kubernetes/community/blob/master/contributors/devel/sig-architecture/api-conventions.md#typical-status-properties
• https://docs.openshift.com/container-platform/4.7/operators/operator_sdk/golang/osdk-golang-tutorial.html
• https://github.com/kubernetes/kubernetes/issues/83094#issuecomment-542721478

Acceptance criteria

• Status conditions are implemented to communicate progress toward reconciling a GitLab CustomResource