Coming Soon: add coming soon to the page selector

[ramonjd]

If a user deletes their editable coming soon page we'll need a way for them to reinstate it.

One suggestion is to add a simple coming soon template to the page picker.

When selected we'll update the WP option to the new ID.

[roo2]

I'm currently working on getting the me/sites endpoint to return wpcom_public_coming_soon_page_id correctly when it is set which will involve updating the D45700-code diff.

I see there is code to record changing the page_on_front etc in the activity stream so I will also make sure that changing the coming soon page is recorded in the same way as part of this card.

also there is code copying page_on_front across in headstart, I might not have to deal with that that as part of this card but we'll probably need somoething similar for wpcom_public_coming_soon_page_id