Attachments: store mapping from Redmine URLs to GitLab URLs

For #30 (closed) and #48 (closed), I'll need a mapping from a Redmine attachment URL (e.g. https://redmine.tails.boum.org/code/attachments/download/1926/asp-flow-removed.svg) to the URL of the corresponding attachment that was imported on GitLab.

It would be good enough for me if redlab upload saved this mapping in a text file with a structured format (ideally: YAML).