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)

model/post.go

@@ -20,6 +20,7 @@ const (
 	POST_JOIN_LEAVE            = "system_join_leave" // Deprecated, use POST_JOIN_CHANNEL or POST_LEAVE_CHANNEL instead
 	POST_JOIN_CHANNEL          = "system_join_channel"
 	POST_LEAVE_CHANNEL         = "system_leave_channel"
+	POST_JOIN_TEAM             = "system_join_team"
 	POST_LEAVE_TEAM            = "system_leave_team"
 	POST_ADD_REMOVE            = "system_add_remove" // Deprecated, use POST_ADD_TO_CHANNEL or POST_REMOVE_FROM_CHANNEL instead
 	POST_ADD_TO_CHANNEL        = "system_add_to_channel"
@@ -169,13 +170,27 @@ func (o *Post) IsValid() *AppError {
 		return NewAppError("Post.IsValid", "model.post.is_valid.hashtags.app_error", nil, "id="+o.Id, http.StatusBadRequest)
 	}
 
-	if !(o.Type == POST_DEFAULT || o.Type == POST_JOIN_LEAVE || o.Type == POST_ADD_REMOVE ||
-		o.Type == POST_JOIN_CHANNEL || o.Type == POST_LEAVE_CHANNEL || o.Type == POST_LEAVE_TEAM ||
-		o.Type == POST_REMOVE_FROM_CHANNEL || o.Type == POST_ADD_TO_CHANNEL || o.Type == POST_ADD_TO_TEAM ||
-		o.Type == POST_SLACK_ATTACHMENT || o.Type == POST_HEADER_CHANGE || o.Type == POST_PURPOSE_CHANGE ||
-		o.Type == POST_DISPLAYNAME_CHANGE || o.Type == POST_CHANNEL_DELETED ||
-		strings.HasPrefix(o.Type, POST_CUSTOM_TYPE_PREFIX)) {
-		return NewAppError("Post.IsValid", "model.post.is_valid.type.app_error", nil, "id="+o.Type, http.StatusBadRequest)
+	switch o.Type {
+	case
+		POST_DEFAULT,
+		POST_JOIN_LEAVE,
+		POST_ADD_REMOVE,
+		POST_JOIN_CHANNEL,
+		POST_LEAVE_CHANNEL,
+		POST_LEAVE_TEAM,
+		POST_REMOVE_FROM_CHANNEL,
+		POST_ADD_TO_CHANNEL,
+		POST_ADD_TO_TEAM,
+		POST_JOIN_TEAM,
+		POST_SLACK_ATTACHMENT,
+		POST_HEADER_CHANGE,
+		POST_PURPOSE_CHANGE,
+		POST_DISPLAYNAME_CHANGE,
+		POST_CHANNEL_DELETED:
+	default:
+		if !strings.HasPrefix(o.Type, POST_CUSTOM_TYPE_PREFIX) {
+			return NewAppError("Post.IsValid", "model.post.is_valid.type.app_error", nil, "id="+o.Type, http.StatusBadRequest)
+		}
 	}
 
 	if utf8.RuneCountInString(ArrayToJson(o.Filenames)) > POST_FILENAMES_MAX_RUNES {