MDL-55074 theme_boost: Navigation and blocks

MDL-55074 theme_boost: Navigation and blocks

This patch includes a big set of changes that are all designed to work together to provide
a better way to navigate in the new theme, and a different way of working with blocks.

Blocks have been moved to a "drawer" that can be opened and closed (this is remembered in a user pref).

A new "flat navigation" element is also available in a drawer - which should let you do 90% of things
without needing to open the "blocks" drawer.

The flat navigation is build from specific parts of the nav tree - the top nodes like "calendar, dashboard" are
hand picked. There is a mycourses node listing your enrolled courses.

There is a node for the current course, built from the top nodes in the current course node in the nav tree.

Administrators have a link to the Site admin settings here too.

These nav elements are used by the templates for the new theme, which also has a resigned layout for login and signup.

There have also been some additional fixes / improvements to the scss for the new theme which goes along with these
layout changes.

This set of changes is a collaboration between Martin, Damyon and Alberto (thanks!).