Set link to favicon to get rid of `RuntimeException: no resource`

Without this change the console shows this error:

```
[debug] application - Unforseen error for favicon.ico at /public
java.lang.RuntimeException: no resource
        at controllers.Assets$$anonfun$controllers$Assets$$assetInfoFromResource$1$$anonfun$10.apply(Assets.scala:214) ~[play_2.10-2.3.0.jar:2.3.0]
        at controllers.Assets$$anonfun$controllers$Assets$$assetInfoFromResource$1$$anonfun$10.apply(Assets.scala:214) ~[play_2.10-2.3.0.jar:2.3.0]
        at scala.Option.getOrElse(Option.scala:120) [scala-library.jar:na]
        at controllers.Assets$$anonfun$controllers$Assets$$assetInfoFromResource$1.apply(Assets.scala:214) ~[play_2.10-2.3.0.jar:2.3.0]
        at controllers.Assets$$anonfun$controllers$Assets$$assetInfoFromResource$1.apply(Assets.scala:213) ~[play_2.10-2.3.0.jar:2.3.0]
```