Update Font Awesome from 6.5.2 to 6.6.0. (#924)

### 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

Update version of Font Awesome used by the project from 6.5.2 to 6.6.0.
Note that this both adds some new icons and also updates a couple
(including the soundcloud logo).

You can find the changelog at https://fontawesome.com/changelog#v6-6-0,
but be aware that some of the changes mentioned here are not included in
Free, only in Pro.

These files have all been directly unpacked from the free ZIP on the
Font Awesome website and then copied directly into the appropriate
directories. No file has been modified.

### Issues Resolved

None

### Checklist

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

#### 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

- [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