start to flesh out GitLab labels and issue drafting

Move the labels and review discussion out of the workflow reference and into appropriate reference and explanation guides.

Add in the ability to render GitLab labels as they appear on GitLab. This provides greater visual similarity between GitLab and the documentation.

Part of ongoing work #188 (closed). Opening here so that people can provide feedback on https://guidebook.devops.uis.cam.ac.uk/en/restructure/reference/gitlab-labels/#spikes.