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.

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,
 	}

pkg/login/ldap_login.go

@@ -51,6 +51,7 @@ var loginUsingLDAP = func(query *models.LoginUserQuery) (bool, error) {
 	}
 
 	upsert := &models.UpsertUserCommand{
+		ReqContext:    query.ReqContext,
 		ExternalUser:  externalUser,
 		SignupAllowed: setting.LDAPAllowSignup,
 	}

pkg/services/quota/quota.go

@@ -23,7 +23,12 @@ func (qs *QuotaService) QuotaReached(c *m.ReqContext, target string) (bool, erro
 	if !setting.Quota.Enabled {
 		return false, nil
 	}
-
+	// No request context means this is a background service, like LDAP Background Sync.
+	// TODO: we should replace the req context with a more limited interface or struct,
+	//       something that we could easily provide from background jobs.
+	if c == nil {
+		return false, nil
+	}
 	// get the list of scopes that this target is valid for. Org, User, Global
 	scopes, err := m.GetQuotaScopes(target)
 	if err != nil {