PLT-7849 Add config setting to disable plugin uploads (#7666)

* Add config setting to disable plugin uploads

* Update unit test

i18n/en.json

@@ -3489,7 +3489,11 @@
   },
   {
     "id": "app.plugin.disabled.app_error",
-    "translation": "Plugins have been disabled by the system admin or the server has not been restarted since they were enabled."
+    "translation": "Plugins have been disabled."
+  },
+  {
+    "id": "app.plugin.upload_disabled.app_error",
+    "translation": "Plugins and/or plugin uploads have been disabled."
   },
   {
     "id": "app.plugin.extract.app_error",