[MM-30539] New renewal link logic (#16539)

* New renewal link logic

Endpoint and logic that returns the renewal link to be used to start the
license renewal process.

* Limit access for restricted sysadmins

* Include active users in the renewal token

model/system.go

@@ -14,6 +14,7 @@ const (
 	SYSTEM_RAN_UNIT_TESTS                         = "RanUnitTests"
 	SYSTEM_LAST_SECURITY_TIME                     = "LastSecurityTime"
 	SYSTEM_ACTIVE_LICENSE_ID                      = "ActiveLicenseId"
+	SYSTEM_LICENSE_RENEWAL_TOKEN                  = "LicenseRenewalToken"
 	SYSTEM_LAST_COMPLIANCE_TIME                   = "LastComplianceTime"
 	SYSTEM_ASYMMETRIC_SIGNING_KEY                 = "AsymmetricSigningKey"
 	SYSTEM_POST_ACTION_COOKIE_SECRET              = "PostActionCookieSecret"