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/syncthing/core/aboutModalView.html

@@ -1,4 +1,4 @@
-<modal id="about" large="yes" close="yes" status="info" title="{{'About' | translate}}">
+<modal id="about" status="info" icon="heart-o" title="{{'About' | translate}}" large="yes" close="yes">
   <h1 class="text-center"><img alt="Syncthing" title="Syncthing" src="assets/img/logo-horizontal.svg" style="vertical-align: -16px" height="100" width="366"/><br/><small>{{versionString()}}</small></h1>
   <hr/>
 
@@ -86,5 +86,6 @@
     <li><a href="https://github.com/vitrun/qart">vitrun/qart</a>, Copyright &copy; The Go Authors.</li>
     <li><a href="https://angularjs.org/">AngularJS</a>, Copyright &copy; 2010-2015 Google, Inc.</li>
     <li><a href="http://getbootstrap.com/">Bootstrap</a>, Copyright &copy; 2011-2015 Twitter, Inc.</li>
+    <li><a href="http://fontawesome.io/">Font Awesome</a>, Copyright &copy; 2015 Dave Gandy</li>
   </ul>
 </modal>