PLT-6217-bis Fixes system message posted to Town Square when a user joins a team #7863 (#7864)

* Adds new string post message when a user has joined a team (#6483)

* Simplifies long if statement in post.go (#6483)

i18n/en.json

@@ -2286,6 +2286,10 @@
     "id": "api.team.is_team_creation_allowed.domain.app_error",
     "translation": "Email must be from a specific domain (e.g. @example.com). Please ask your systems administrator for details."
   },
+  {
+    "id": "api.team.join_team.post_and_forget",
+    "translation": "%v joined the team."
+  },
   {
     "id": "api.team.leave.left",
     "translation": "%v left the team."