In https://github.com/discourse/discourse-solved/pull/342 we moved solutions away from topic_custom_fields into proper tables, with the tables as the proper source of truth to a topic's solution.
The user's /my/activity/solved route uses user_actions which is not accurate, and a user has reported a bug where their solution is not reflected there (user actions are not a good representation of what a topic's solution is).
This commit introduces
- a new route to get solutions, and is mindful `hide_user_profiles_from_public` and such settings
- also mindful of PMs and private categories
- a new template that makes use of the `<UserStream>` to load posts safely and avoid reimplementation
