New secondary menu

Hey all. First pass at the secondary menu ready for you. Here's what I did:

* Add secondary-menu html partial and scss. Create new jekyll layout for page-interior to show it.
* Fix margin and padding on .identity in site header to get rid of weird gap between site header and secondary menu.
* Adjust position of "." that is added after "Common Core Success" in the site header so it looks right.

I didn't write anything that, for example, converts this to a select menu on mobile. Haven't tackled that one before and wanted to see what your take was on that kind of approach.