I’ve just revisited the status of this plugin from this topic:
https://meta.discourse.org/t/installing-sidebar-plugin/34101
The plugin works for us in Discourse 1.5 (thanks to the PR from @zogstrip) but it seems it only shows basic html as @zajako said, and yes, the box is one-liner.
This plugin was released by our organization (Tc) via paying a developer team (@Daniel_Fernandez_Pes) to serve a specific project but we were not able to continue it with latest releases of Discourse (no time nor budget enough) so PRs are welcome to solve these issues (it’s GPL license) 
In mobile it doesn’t show the sidebar.