[bug 987331] Add under construction template

This adds an under construction template. This lets us test it out and
then we'll swap it in for the front page (or not) later.
[bug 987331] Fix under construction placeholder
Fix front page dashboard

* tweaks some things so we don't end up with an empty tomorrow in the
  graph anymore
* add weekend markers
* add note about the upcoming product dashboard overhaul with a link to
  the survey to get some more insight on what other people wish the
  dashboard could do