Update RFC8424 discovery with structured LOLA metadata

[aaronjae22]

This PR updates RFC8414 Discovery Endpoint with structured LOLA account portability metadata.

Before:

"activitypub_account_portability": "<http://localhost:8000/oauth/authorize/>"

After:

"activitypub_account_portability": {
  "supported": true,
  "authorization_endpoint": "<http://localhost:8000/oauth/authorize/>",
  "scopes": ["activitypub_account_portability"]
}

curl -s http://localhost:8000/.well-known/oauth-authorization-server | jq

Closes #240