Embedded images broken if user not logged into Confluence

[Andrew S]

Steps to reproduce:

1. Use this plugin to create a Confluence page for a JIRA issue.
2. Drag an image into that page so that it's shown as an image attachment.
3. Log out of Confluence.
4. In JIRA, open the Confluence tab for that issue; the image is broken (shows a browser-specific "broken image" icon).
5. Log back into Confluence.
6. Reload the Confluence tab for the issue; the image displays correctly.

[Andrew S]

The initial loading of the Confluence page content is done using the credentials stored against the application link. However the browser cannot use these credentials when requesting any Confluence URLs that appear within the iframe.

[Andrew S]

Given that it's possible for a JIRA user to have a Confluence OAuth token but not be logged into Confluence (e.g. they may have gone to a different machine), any requests to Confluence URLs (e.g. for embedded images) will probably need to be proxied by my plugin to ensure they succeed. This will involve modifying the page HTML so that any Confluence URLs are changed to plugin URLs. Jsoup looks like a good way to do this.