Missing css links and gateway.template needs updating

In webwork2/htdocs the symlinks

```
 math.css, math2.css, math3.css, union.css and ur.css
```

point to locations which no longer exist.  

Also, gateway.template tries to include 

```
htdocs/css/math.css and htdocs/css/gateway.css.  
```

But these files are missing.  They are not in htdocs/css and they are not in any of the hdocs/themes subdirectories either.  This makes gateways look bad.
