Remove created_at from WikiPage

This MR removes the method created_at from WikiPage because there are several things wrong about it:

• we don't call it anywhere.
• it has a bug. It calls version.date and WikiPageVersion doesn't have a method date.
• it is conceptually wrong. It doesn't return the created_at of the page, it'd have returned the date when the last commit in the wiki was made.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary