Send ephemeral message when a mentioned user is not a member of the team (#26153)

Co-authored-by: Caleb Roseland <caleb@calebroseland.com>
Co-authored-by: Carrie Warner (Mattermost) <74422101+cwarnermm@users.noreply.github.com>
Co-authored-by: Sazzad Hossain <sazzad.hossain@marginedge.com>
Co-authored-by: Mattermost Build <build@mattermost.com>

server/i18n/en.json

@@ -2500,6 +2500,14 @@
     "id": "api.post.check_for_out_of_channel_mentions.message.one",
     "translation": "@{{.Username}} did not get notified by this mention because they are not in the channel."
   },
+  {
+    "id": "api.post.check_for_out_of_team_mentions.message.multiple",
+    "translation": "@{{.Usernames}} and @{{.LastUsername}} didn't get notified by this mention because they aren't members of this team."
+  },
+  {
+    "id": "api.post.check_for_out_of_team_mentions.message.one",
+    "translation": "@{{.Username}} didn't get notified by this mention because they aren't a member of this team."
+  },
   {
     "id": "api.post.create_post.can_not_post_to_deleted.error",
     "translation": "Can not post to deleted channel."