Native sign-in with multi-factor authentication (MFA)

# About

Native sign-in with MFA allows for the user to own their account and data with maximum security. This aligns with the open-source values of HackMD.

# User flow

1. Create a HackMD account with a native username, password, and email. (Not using a third-party sign in like Twitter or GitHub).
2. Under *Settings*, set up multi-factor authentication (MFA) or two-factor authentication (TFA)
a. Choose type of MFA: FIDO2 with a hardware device like a Yubikey or OATH-TOTP with an app like Yubico Authenticator, email, or phone number.

# Alternative

**Migrate an account's native sign-in from username and password to third-party sign-in for MFA**
- By switching from native sign-in to a third-party sign that has MFA setup adds an extra layer of security in case the password is leaked or hacked.
- However, the user then must rely on the third-party service for their entire account data.