Added ephemeral messages sent when a user mentions someone not in the channel

i18n/en.json

@@ -655,6 +655,14 @@
     "id": "api.oauth.revoke_access_token.get.app_error",
     "translation": "Error getting access token from DB before deletion"
   },
+  {
+    "id": "api.post.check_for_out_of_channel_mentions.message.one",
+    "translation": "{{.Username}} was mentioned, but they do not belong to this channel."
+  },
+  {
+    "id": "api.post.check_for_out_of_channel_mentions.message.multiple",
+    "translation": "{{.Usernames}} and {{.LastUsername}} were mentioned, but they do not belong to this channel."
+  },
   {
     "id": "api.post.create_post.bad_filename.error",
     "translation": "Bad filename discarded, filename=%v"
@@ -3459,4 +3467,4 @@
     "id": "web.watcher_fail.error",
     "translation": "Failed to add directory to watcher %v"
   }
-]
+]

i18n/es.json

@@ -655,6 +655,14 @@
     "id": "api.oauth.revoke_access_token.get.app_error",
     "translation": "Error obteniendo el token de acceso desde la BD antes de ser eliminado"
   },
+  {
+    "id": "api.post.check_for_out_of_channel_mentions.message.one",
+    "translation": "{ .Username } was mentioned, but they do not belong to this channel."
+  },
+  {
+    "id": "api.post.check_for_out_of_channel_mentions.message.multiple",
+    "translation": "{ .Usernames } and { .LastUsername} were mentioned, but they do not belong to this channel."
+  },
   {
     "id": "api.post.create_post.bad_filename.error",
     "translation": "Nombre errado de archivo descartado, archivo=%v"
@@ -3459,4 +3467,4 @@
     "id": "web.watcher_fail.error",
     "translation": "Falla al agregar el directorio a ser vigilado %v"
   }
-]
+]