Improve the OpenProject integration extension for xwiki.org

What xwiki.org pages need before their Jira macros can be converted:

• guest access (public data shown to users without an OpenProject account)
• work package keys (XWIKI-123) in the identifier parameter
• list and enum displayers, with closed items struck through
• the Target versions, Observed in versions, Categories, Labels and custom fields

Tracked in the extension's GitHub issues (https://github.com/xwikisas/project-management/issues), linked here.