Treat the links in the footer as external. (#796)

### Prerequisites

Put an `x` into the box(es) that apply:

- [ ] This pull request fixes a bug.
- [x] This pull request adds a feature.
- [ ] This pull request introduces breaking change.

### Description

As of a couple of weeks ago, you added the _blank and noopener
properties to external links inside articles. While the little icon
doesn't really make sense for the footer, the idea of the link to github
or the link to hugo's own website being treated as an external location
(_blank + noopener) makes a lot of sense.

### Issues Resolved



### Checklist

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#### General

- [x] Describe what changes are being made
- [x] Explain why and how the changes were necessary and implemented
respectively
- [ ] Reference issue with `#<ISSUE_NO>` if applicable

#### Resources

- [ ] If you have changed any SCSS code, run `make release` to
regenerate all CSS files

#### Contributors

- [x] Add yourself to `CONTRIBUTORS.md` if you aren't on it already