Display git tags

To trigger a new pipeline via the GitLab Dashboard, we need to fetch the available git tags and display them in a form.

• There's a Translucent form for each app & stage (in the app stage detail view where the pipelines are displayed)
• The form has a dropdown with available versions (i.e. tags)
• The tags are fetched from the GitLab project via API
• It can be selected if the pipeline runs on the tag or default branch