Proof of Mastodon Account Ownership always Fails

To Reproduce:

Create a Mastodon account if you don't have one already. Click 'Add more identities' in the Keybase app (Winx64) and choose your Mastodon instance. You'll be prompted for your username. The proof of ownership process then continues by redirecting the user to a specially crafted URL located at the Mastodon instance.

**That URL always leads to a 404 error.**

It doesn't matter which Mastodon instance is being confirmed. It always fails.

Reproduce the failure through the Keybase app's “Add more identities” flow on Winx64, selecting a Mastodon instance and username. Inspect the generated Mastodon proof URL and the ownership-verification entry point; done means the URL no longer returns 404 and the proof completes for a Mastodon account.

Written by the indexing model from the issue text.