Issue #357 - Display "no results found" when no results are found.

Added generic "no results found" text. @magsout could you add a touch of style to this? It looks a bit squashed at the bottom, if that makes sense.

To reproduce you can either log-in and search for nonsense, or click the "Site Contacted" filter button (http://localhost:5000/issues?sitewait=1) if you have `ISSUES_REPO_URI = "miketaylr/nobody-look-at-this/issues"` in config.py.

![blah](https://cldup.com/kSfQo4RdgR-2000x2000.png)