Enforce two-factor auth (2FA: TOTP or WebAuthn) (#34187)

Fix #880

Design:

1. A global setting `security.TWO_FACTOR_AUTH`.
* To support org-level config, we need to introduce a better "owner
setting" system first (in the future)
2. A user without 2FA can login and may explore, but can NOT read or
write to any repositories via API/web.
3. Keep things as simple as possible.
* This option only aggressively suggest users to enable their 2FA at the
moment, it does NOT guarantee that users must have 2FA before all other
operations, it should be good enough for real world use cases.
* Some details and tests could be improved in the future since this
change only adds a check and seems won't affect too much.

---------

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

modules/setting/security.go

@@ -39,6 +39,7 @@ var (
 	CSRFCookieName                     = "_csrf"
 	CSRFCookieHTTPOnly                 = true
 	RecordUserSignupMetadata           = false
+	TwoFactorAuthEnforced              = false
 )
 
 // loadSecret load the secret from ini by uriKey or verbatimKey, only one of them could be set
@@ -142,6 +143,15 @@ func loadSecurityFrom(rootCfg ConfigProvider) {
 	PasswordCheckPwn = sec.Key("PASSWORD_CHECK_PWN").MustBool(false)
 	SuccessfulTokensCacheSize = sec.Key("SUCCESSFUL_TOKENS_CACHE_SIZE").MustInt(20)
 
+	twoFactorAuth := sec.Key("TWO_FACTOR_AUTH").String()
+	switch twoFactorAuth {
+	case "":
+	case "enforced":
+		TwoFactorAuthEnforced = true
+	default:
+		log.Fatal("Invalid two-factor auth option: %s", twoFactorAuth)
+	}
+
 	InternalToken = loadSecret(sec, "INTERNAL_TOKEN_URI", "INTERNAL_TOKEN")
 	if InstallLock && InternalToken == "" {
 		// if Gitea has been installed but the InternalToken hasn't been generated (upgrade from an old release), we should generate