Web site improvements - Githubissues

Here are some improvements to the web site.

With the latest GitHub Pages update, page.content is now the unrendered markdown, while content is the rendered html.
Update the tags in the <head> section to match Bootstrap's recommendations.
Use a sticky footer so the footer stays at the bottom of the window, even if there isn't enough content to fill the window.
Move the navbar to an include file for easier future editing.

apple / ccs-calendarserver