Build side nav template layout and logic

[afred]

Background

The default layout will have a navigation menu on the left side. (Pages without a nav menu will use a layout other than default).

The template for the menu (i.e. _includes/side_nav.html) will be responsible for rendering a tree of menu data.

Consider using a recursive template to handle arbitrarily deep trees, because we haven't yet decided on what to limit the nav menu tree depth to, if anything.

Done when

• The default layout includes rendering a navigation menu on the left side.
• The _includes/side_nav.html template renders a tree of menu data.
• Style is added to _sass/side_nav.scss to set the following properties (at least):
  • position -- left of main content, below header, above footer
  • width -- side nav should be a consistent width for any given breakpoint. Width should not depend on the content within the nav menu.
  • List style - remove any list style if using <ul>...</ul>
  • Link style - be consistent with site-wide style for color, font, weight, background, text-decoration, and :hover