Confluence thumbnails do not render in JIRA activity stream

Refer to: https://ecosystem.atlassian.net/browse/STRM-2137

Steps to reproduce:
1. Create app link between JIRA and Confluence
2. Add Confluence activity stream gadget to JIRA dashboard
3. Add a new page in Confluence.
4. Insert an image in new page, resize it and save.
5. Refresh Confluence activity stream on JIRA dashboard.
6. Confluence add 2 updates to activity stream for the new page. Latest states an attachment is added, with picture of image. Latest, but one, states new page is added with broken image icon.

Expected Behaviour:
Both images should be rendered fine.

Analysis
Check image location of the 2 images from recent activity. Attachment one is complete url to image in /download/attachments. Page one is relative url to image in /download/thumbnails and is not rendered fine as JIRA tries to resolve it with JIRA base url.

DefaultAttachedImageResourceMarshaller in Confluence does not prepend base url to thumbnail images resulting in broken link in activity stream.