Integration of answers from external source

If the mentions are notified to an external user using an extension mechanism (for instance in the context of an integration of the user mentions with ActivityPub), answers to the notification (for instance by a comment on Mastodon), should be gracefully integrated into the UI.

In summary, answering to mentions should be open to extensions.

See https://design.xwiki.org/xwiki/bin/view/Proposal/UserMentions#HUC11.Integrationofanswersfromexternalsource