Add a workflow_url (like the previous v1 job api job_url) to the /get /pipeline/pipeline_id/workflow api

So we currently use the pipeline api to trigger workflows using a slack bot. We then retrieve the workflows triggered in the pipeline and the bot returns a generated url to the first workflow in that pipeline.

Since there’s no equivalent to job_url from the v1 job api, i have to create the workflow_url myself.

Recently the changes to the workflow url pattern changed and therefore the workflow_url generated by my code no longer worked.