Add mastodon

[OldPanda]

This PR is to add mastodon stats to the repo. The webpage looks like and the stats API responds like

$curl http://127.0.0.1:8787/stats/mastodon/oldpanda@mastodon.social | jq .
{
  "source": "mastodon",
  "key": "oldpanda@mastodon.social",
  "failed": false,
  "count": 4
}

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Name	Status	Preview	Updated
substats	✅ Ready (Inspect)	Visit Preview	Jan 8, 2023 at 4:34PM (UTC)

[spencerwooo]

I have no idea why the staging action cannot pick up CF_API_TOKEN from the repo's secrets. Merging as-is.

[OldPanda]

Thanks!