Quota: Makes sure we provide the request context to the quota service (#21949)

It was missing for ldap_login which means that the first signup failed
for users with LDAP+quota enabled. There's also potential cases where we
can't provide a request context (background jobs) which is also covered,
but needs a refactoring.

(cherry picked from commit 59530e4758)

pkg/api/ldap_debug.go

@@ -216,6 +216,7 @@ func (server *HTTPServer) PostSyncUserWithLDAP(c *models.ReqContext) Response {
 	}
 
 	upsertCmd := &models.UpsertUserCommand{
+		ReqContext:    c,
 		ExternalUser:  user,
 		SignupAllowed: setting.LDAPAllowSignup,
 	}