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>

services/forms/auth_form.go

@@ -14,9 +14,11 @@ import (
 
 // AuthenticationForm form for authentication
 type AuthenticationForm struct {
-	ID                            int64
-	Type                          int    `binding:"Range(2,7)"`
-	Name                          string `binding:"Required;MaxSize(30)"`
+	ID              int64
+	Type            int    `binding:"Range(2,7)"`
+	Name            string `binding:"Required;MaxSize(30)"`
+	TwoFactorPolicy string
+
 	Host                          string
 	Port                          int
 	BindDN                        string
@@ -74,7 +76,6 @@ type AuthenticationForm struct {
 	Oauth2RestrictedGroup         string
 	Oauth2GroupTeamMap            string `binding:"ValidGroupTeamMap"`
 	Oauth2GroupTeamMapRemoval     bool
-	SkipLocalTwoFA                bool
 	SSPIAutoCreateUsers           bool
 	SSPIAutoActivateUsers         bool
 	SSPIStripDomainNames          bool