Lesson components and some more general tweaks/fixes

Lots of changes all in one commit. Apologies for the long-ass commit message.

## On the site header and general page template:

* Added header-interior.html which gets rid of big message for interior pages in favor of section title
* Added '.' into markup of header.html instead of adding it via :after pseudo-classes

## For the lesson detail pages:

* Added placeholder icon font so we can start to test implementation
* Changed layout template for lesson page to page-interior
* Added some classed spans to pager markup to make it easier to work with internal pager link elements
* Added partials for _detail-page-header.scss, _icons.scss, _pager.scss, _jump-links.scss with styling for all
* Moved node-bottom on lesson detail page so it will clear the primary and secondary columns and span full-width

More to refine on lesson detail page, but this should give us a start.