Build layout and logic for version selector

[afred]

Description

each document in the _software collection should have a specific version that the user must be allowed to switch to.

Done when

• The software layout includes rendering the version selector.
• The _includes/versions.html template renders a list of available versions for the current software. (Other software/versions should be accessible from a top nav menu).
• Style is added to _sass/versions.scss to set the following properties (at least):
  • position -- somewhere in a conspicuous location, like across the top? Or maybe top right?
  • size -- height and width should be set to provide a consistent experience. Don't allow the positioning of main content to be affected by the number of versions.
  • Link/Button style - be consistent with site-wide styles for color, font, weight, background, text-decoration, :hover, etc