Extend [skip ci] to work with tags

Customer ticket: https://gitlab.zendesk.com/agent/tickets/104005

At the moment, adding [skip ci] or [ci skip] to a commit message will skip the CI pipeline for that commit. We should extend this functionality to tags as well. At the moment, adding either of these values to the tag message is ignored so the pipeline is still run.

Proposal: We should skip the pipeline if a tag has either [skip ci] or [ci skip] in the tag message.