Create the 'About discussion' widget

There should be an aboutdiscussion widget that has 2 different tabs. The first tab is the Details tab and shows the metadata of the discussion (title, description, created by). When a description is not available, it should not be shown. The second tab is the Shared with tab and shows the list of members of the discussion.
Looks like the `h3` doesn't have the expected `discussionshared-modal-title` id. This is true for the other metadata widgets as well. I'll follow up.