add pseudo-"Article Bottom" widget area to pages

[benlk]

Copied from #42 (comment)

This is meant to complement the "Article Bottom" widget area, and should be called something like "Page Bottom", with description text that emphasizes that it shows up on all pages

• shouldn't probably be used for the newsletter, because the newsletter is in the main sidebar?

[kaylima]

Sorry, I don't follow. What's this widget area for?

[benlk]

Oh, this was mentioned during review at some point. We wanted an "Article bottom"-equivalent widget area for pages, since that widget area only outputs on posts.