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/upgradingDialogView.html

@@ -1,3 +1,3 @@
-<modal id="upgrading" icon="refresh" title="{{'Upgrading' | translate}}" status="info">
+<modal id="upgrading" status="info" icon="arrow-circle-up" title="{{'Upgrading' | translate}}">
 	<p><span translate>Syncthing is upgrading.</span> <span translate>Please wait</span>...</p>
 </modal>