PLT-2674 Private Group should not have Leave option when only one member remains (#2888)

i18n/en.json

@@ -223,6 +223,10 @@
     "id": "api.channel.leave.direct.app_error",
     "translation": "Cannot leave a direct message channel"
   },
+  {
+    "id": "api.channel.leave.last_member.app_error",
+    "translation": "You're the only member left, try removing the Private Group instead of leaving."
+  },
   {
     "id": "api.channel.leave.left",
     "translation": "%v has left the channel."

i18n/es.json

@@ -223,6 +223,10 @@
     "id": "api.channel.leave.direct.app_error",
     "translation": "No puedes dejar un mensaje directo a un canal"
   },
+  {
+    "id": "api.channel.leave.last_member.app_error",
+    "translation": "Eres el último miembro que queda, intenta remover el Grupo Privado en vez de salirte."
+  },
   {
     "id": "api.channel.leave.left",
     "translation": "%v ha abandonado el canal."