#768 - Add feature toggle for plugin upload form.

To turn this on, you need to do this:

```
curl -d 'toggle_value=on' 'http://go_server/go/api/admin/feature_toggles/plugin_upload_feature_toggle_key'
```

When the Go server has authentication turned on, use with the --user option, like this:

```
curl --user username:password -d 'toggle_value=on' ...
```