Reopen Side Peek on last post

**Is your feature request related to a problem? Please describe.**
If I view a post on the Side Peek, and then close SP to go back to the timeline, I can't simply reopen the last post. For example if I looked for a specific post through my notifications, maybe browsed some posts on the way, I'll have to go by the same path again to find that post.

**Describe the solution you'd like**
A button that will re-open the Side Peek pane to the last post that was viewed.

Thanks!

Start by locating the Side Peek implementation and how it tracks the currently viewed post. Add a way from the timeline to reopen the last viewed post, then verify that closing and reopening Side Peek restores that post, including when it was reached through notifications.

Written by the indexing model from the issue text.