Map OIDC groups to Orgs/Teams (#21441)

Fixes #19555

Test-Instructions:
https://github.com/go-gitea/gitea/pull/21441#issuecomment-1419438000

This PR implements the mapping of user groups provided by OIDC providers
to orgs teams in Gitea. The main part is a refactoring of the existing
LDAP code to make it usable from different providers.

Refactorings:
- Moved the router auth code from module to service because of import
cycles
- Changed some model methods to take a `Context` parameter
- Moved the mapping code from LDAP to a common location

I've tested it with Keycloak but other providers should work too. The
JSON mapping format is the same as for LDAP.


![grafik](https://user-images.githubusercontent.com/1666336/195634392-3fc540fc-b229-4649-99ac-91ae8e19df2d.png)

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

services/forms/auth_form.go

@@ -72,13 +72,15 @@ type AuthenticationForm struct {
 	Oauth2GroupClaimName          string
 	Oauth2AdminGroup              string
 	Oauth2RestrictedGroup         string
+	Oauth2GroupTeamMap            string `binding:"ValidGroupTeamMap"`
+	Oauth2GroupTeamMapRemoval     bool
 	SkipLocalTwoFA                bool
 	SSPIAutoCreateUsers           bool
 	SSPIAutoActivateUsers         bool
 	SSPIStripDomainNames          bool
 	SSPISeparatorReplacement      string `binding:"AlphaDashDot;MaxSize(5)"`
 	SSPIDefaultLanguage           string
-	GroupTeamMap                  string
+	GroupTeamMap                  string `binding:"ValidGroupTeamMap"`
 	GroupTeamMapRemoval           bool
 }