Add support for themes (fixes #1925)

gui/default/syncthing/device/idqrModalView.html

@@ -0,0 +1,4 @@
+<modal id="idqr" status="info" icon="qrcode" title="{{'Device Identification' | translate}} - {{deviceName(currentDevice)}}" large="yes" close="yes">
+  <div class="well well-sm text-monospace text-center" select-on-click>{{currentDevice.deviceID}}</div>
+  <img ng-if="currentDevice.deviceID" class="center-block img-thumbnail" ng-src="qr/?text={{currentDevice.deviceID}}"/>
+</modal>