Disable autoupgrade if not supported (fixes #763)

I can swap them over, but then it looks like this:
```
 [ ] Global disco  [ ] Local disco
 [ ] UPnP          [ ] Upgrades

Global announce server:
[announce.syncthing.net_______________________]
```

which is a bit backwards.
Also, hiding the whole div does the following:
```
 [ ] Global disco  [ ] Local disco
 [ ] UPnP          Global announce server:
 [announce.syncthing.net_______________________]
```

Perhaps there is some magic whereby we can get rid of the whole div but still preserve the space it takes (I guess opacity:0) but that doesn't mean that we'll be able to use that space once other options appear.