Implement Mastodon comments (#802)

### 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

Implements Mastodon comments with Carl Schwan solution:
https://carlschwan.eu/2020/12/29/adding-comments-to-your-static-blog-with-mastodon/

This allow to link post with a Mastodon toot, comments on the Mastodon
toot will be displayed on the post footer.

### 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

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