Guide page picker order of pages

**Acceptance criteria**
- [ ] On page pickers within a guide: the pages display in reverse chronological order according to what was **most recently edited**

As a user, it's more likely that I'm using fresher content than working with older content. When I am selecting a page for a guide, or selecting a topic for a page, the view of eligible items should show by newest creation first. (Alternatively, we could allow column level sorts, but in that case would still want most recent content as the default order.)