Add G+ integration

We have to link to G+ on our site to get it fully integrated with Google's search.

The link looks like:

```
Google+
```

However, I wonder if there's a badge that we should put instead.

Start by locating the site homepage or template that contains external links and inspect how similar integrations are presented. Confirm whether the requested Google+ publisher link or a badge is expected; done means the chosen integration is visible on the site and matches Google's requirement.

Written by the indexing model from the issue text.