Bug 1070499 - Add the URLs to release notes in the release view.

NIT: generally, I prefer to avoid the \ escapes to do a new line, and instead use parens, like so:
```python
return u'<a href="%s/firefox/os/notes/%s/">Staging</a> / <a href="%s/firefox/os/notes/%s/">Public</a>' % (
    (base_url_staging, obj.version, base_url_prod, obj.version)
```

Another suggestion to make this slightly more readable and forward compatible would be to use https://docs.python.org/2/library/string.html#formatstrings instead, something like:

```python
return ('<a href="{staging}/firefox/os/notes/{version}/">Staging</a> / '
        '<a href="{prod}/firefox/os/notes/{version}/">Public</a>'.format(
        staging=base_url_staging, product=product, version=obj.version, prod=base_url_prod))
```