ZeroNet shouldn't be re-running coffee on the pre-installed files.

I am seeing this error in the log:

```
[2016-05-31 17:35:18,125] DEBUG - Running: "/usr/local/bin/coffee" --no-header -p "src/Ui/media/lib/ZeroWebsocket.coffee" (Done in 0.25s)
[2016-05-31 17:35:18,305] DEBUG - Running: "/usr/local/bin/coffee" --no-header -p "src/Ui/media/lib/jquery.csslater.coffee" (Done in 0.18s)
[2016-05-31 17:35:18,306] ERROR - UiWSGIHandler error: IOError: [Errno 13] Permission denied: 'src/Ui/media/all.js' in UiServer.py line 39 > pywsgi.py line 871 > pywsgi.py line 860 > SidebarPlugin.py line 31 > UiRequest.py line 378 > DebugMedia.py line 122
```

ZeroNet is attempting to overwrite the read-only installed file all.js and fails. It first runs the coffee command on the installed file src/Ui/media/lib/ZeroWebsocket.coffee. ZN shouldn't be re-running coffee on the installed files.