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

i18n/en.json

@@ -1090,6 +1090,10 @@
     "id": "api.context.permissions.app_error",
     "translation": "You do not have the appropriate permissions"
   },
+  {
+    "id": "api.context.server_busy.app_error",
+    "translation": "Server is busy, non-critical services are temporarily unavailable"
+  },
   {
     "id": "api.context.session_expired.app_error",
     "translation": "Invalid or expired session, please login again."
@@ -2806,6 +2810,10 @@
     "id": "api.websocket_handler.invalid_param.app_error",
     "translation": "Invalid {{.Name}} parameter"
   },
+  {
+    "id": "api.websocket_handler.server_busy.app_error",
+    "translation": "Server is busy, non-critical services are temporarily unavailable"
+  },
   {
     "id": "app.admin.test_email.failure",
     "translation": "Connection unsuccessful: {{.Error}}"