Reimplement external link icon using Fork Awesome (#791)

### Prerequisites

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

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

### Description

This PR re-implements #779 using Fork Awesome's icon for the external
link instead of a unicode character that may or may not be in the font
being used on the website. Since Fork Awesome is always included in the
CSS and is used elsewhere on the pages, we know this character will
definitely be available and will be consistent with the styling of the
rest of the website.

### Issues Resolved

None.

### 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
- [x] Reference issue with `#<ISSUE_NO>` if applicable

#### Resources

- [x] 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