Replace markdown.js with showdown.js.

Markdown.js wasn't handling inline HTML or quotes correctly. Showdown
seems like a more mature implementation of the markdown syntax.
Add bootstrap css.
Remove ajax spinner styles.
Add bootstrap-responsive.
Fix incorrect link header construction logic.
Add tag manager plugin.
Add sticky footer styles.
Add bootstrap classes to mustache templates.
Change button text while action is being performed.
Add tag autocompletion.