MM-8607 Add ability to turn off non-critical services when under load (#13212)

* MM-8607: add ability to turn off non-critical services under load

* server busy invalid param unit tests

* MM-8607: rename server busy endpoints

* MM-8607: handle case where App not initialized

* MM-8607: additional unit test cases per feedback.

* MM-8607: use decorator to check isbusy when adding endpoint route

* MM-8607: rename endpoints, use struct for json

* Update api4/system.go

Fix misspelled log output

Co-Authored-By: Saturnino Abril <saturnino.abril@gmail.com>

* MM-8607: fix i18n order; max seconds for server busy expiry

api4/apitestlib.go

@@ -636,6 +636,11 @@ func CheckInternalErrorStatus(t *testing.T, resp *model.Response) {
 	checkHTTPStatus(t, resp, http.StatusInternalServerError, true)
 }
 
+func CheckServiceUnavailableStatus(t *testing.T, resp *model.Response) {
+	t.Helper()
+	checkHTTPStatus(t, resp, http.StatusServiceUnavailable, true)
+}
+
 func CheckErrorMessage(t *testing.T, resp *model.Response, errorId string) {
 	t.Helper()