Add sidebar_title to InteractivePage models.

starts: [#64061442]
Authors can edit the title of the tab on the right of the page.
Currently it always is called "Did you know?" slide-out panel.

https://www.pivotaltracker.com/story/show/64061442
Edit interface, and dynamic display title for "Did you know?" sidebar.

complets: [#64061442]
Authors can edit the title of the tab on the right of the page.
Currently it always is called "Did you know?" slide-out panel.

https://www.pivotaltracker.com/story/show/64061442