Pages: show scheduled pages and fix empty placeholders

[gwwar]

Fixes #452, to display scheduled pages.

Testing Instructions:

1. Navigate to http://calypso.localhost:3000/page
2. Add some content to the page editor
3. Schedule the page to be published in the future by clicking on the calendar icon, selecting a future date, and clicking 'Schedule'
4. Navigate to http://calypso.localhost:3000/pages
5. Scheduled tab should be visible
6. Clicking on the Scheduled tab should display your scheduled pages

Before:

After:

cc @alisterscott @rralian

[rralian]

This looks good to me. The unfortunate thing is that /pages has not yet implemented post-counts and so statuses are not conditionally displayed. So scheduled will show up even when there are no scheduled pages. And scheduling pages is pretty uncommon.

But that's not the fault of this PR or something I think we should address here. Pages could use quite a bit of work all around. I think for now it makes sense to just address the reported issue as done here and then circle back at some point to finish implementing conditional statuses for pages.

That's a lot of words to say . 😄

[alisterscott]

Also tested this locally, looks good to me, besides always seeing scheduled which as mentioned will be another fix.

[gwwar]

👍 Yup I noticed that, but it seemed way out of scope for this PR.

I've created #1168 so we can circle back to those enhancements.