Switch to Webmention-based comments

[florimondmanca]

• Closes #160

This PR drops Utterances and switches to Webmention-based comments, powered by Bridgy and Webmention.io.

TODO

• [ ] Submit CSS reset fix as a separate PR
• [ ] Draft a blog post to summarize how this was implemented.
• [ ] Add a TTL on the data fetching cache, so that comments can be updated every once in a while.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (d298858) 99.77% compared to head (526fe55) 99.79%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #484 +/- ## ========================================== + Coverage 99.77% 99.79% +0.01% ========================================== Files 35 36 +1 Lines 891 961 +70 ========================================== + Hits 889 959 +70 Misses 2 2 ```

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