- For WebAuthn Credential level 3, the `backup_eligible` and
`backup_state` flags are checked if they are consistent with the values
given on login. Forgejo never stored this data, so add a database
migration that makes all webauthn credentials 'legacy' and on the next
first use capture the values of `backup_eligible` and `backup_state`.
As suggested in https://github.com/go-webauthn/webauthn/discussions/219#discussioncomment-10429662
- Adds unit tests.
- Add E2E test.
