PLT-2978: Add channel purpose change system message. (#5094)

Completes original patch by David Lu.

model/post.go

@@ -18,6 +18,7 @@ const (
 	POST_ADD_REMOVE            = "system_add_remove"
 	POST_HEADER_CHANGE         = "system_header_change"
 	POST_DISPLAYNAME_CHANGE    = "system_displayname_change"
+	POST_PURPOSE_CHANGE        = "system_purpose_change"
 	POST_CHANNEL_DELETED       = "system_channel_deleted"
 	POST_EPHEMERAL             = "system_ephemeral"
 	POST_FILEIDS_MAX_RUNES     = 150
@@ -120,7 +121,7 @@ func (o *Post) IsValid() *AppError {
 
 	// should be removed once more message types are supported
 	if !(o.Type == POST_DEFAULT || o.Type == POST_JOIN_LEAVE || o.Type == POST_ADD_REMOVE ||
-		o.Type == POST_SLACK_ATTACHMENT || o.Type == POST_HEADER_CHANGE ||
+		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) {
 		return NewLocAppError("Post.IsValid", "model.post.is_valid.type.app_error", nil, "id="+o.Type)
 	}