Add plugin slash command support (#7941)

* add plugin slash command support

* remove unused string

* rebase

i18n/en.json

@@ -4266,6 +4266,10 @@
     "id": "mattermost.working_dir",
     "translation": "Current working directory is %v"
   },
+  {
+    "id": "model.plugin_command.error.app_error",
+    "translation": "An error occurred while trying to execute this command."
+  },
   {
     "id": "model.plugin_key_value.is_valid.plugin_id.app_error",
     "translation": "Invalid plugin ID, must be more than {{.Min}} and a of maximum {{.Max}} characters long."