Switch to Webmention-based comments

[florimondmanca]

• Closes Consider alternative comment systems #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]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.79%. Comparing base (d298858) to head (526fe55).
⚠️ Report is 8 commits behind head on main.
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Additional details and impacted files

@@            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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