Change to Font Awesome icon font (fixes #2138)

- remove Glyphicon assets and customize bootstrap CSS
- add Font Awesome v4.4.0 assets
- replace Glyphicons with Font Awesome icons in HTML
- add icons to modal headers
- add attribution for Font Awesome
- format HTML source code for buttons

gui/vendor/bootstrap/js/bootstrap.js

@@ -5,8 +5,8 @@
  */
 
 /*!
- * Generated using the Bootstrap Customizer (http://getbootstrap.com/customize/?id=4012f96c807ffd49c697)
- * Config saved to config.json and https://gist.github.com/4012f96c807ffd49c697
+ * Generated using the Bootstrap Customizer (http://getbootstrap.com/customize/?id=7ee3aa2c980c52345937)
+ * Config saved to config.json and https://gist.github.com/7ee3aa2c980c52345937
  */
 if (typeof jQuery === 'undefined') {
   throw new Error('Bootstrap\'s JavaScript requires jQuery')