Fix image locations in footer

These are missing `/static/` in the path which was lost on import to django template.

```
GET https://registry.threesixtygiving.org/images/360-giving-logo-white.svg 404 (Not Found)
GET https://registry.threesixtygiving.org/images/github-logo.svg 404 (Not Found)
GET https://registry.threesixtygiving.org/images/twitter-logo.svg 404 (Not Found)
```