MM-11703: Update incoming and outgoing webhook description to 500 characters (#9265)

* MM-11703: Update incoming and outgoing webhook description to 500 characters

* MM-11703: Add upgrade code

* MM-11703: Remove MAX_DESCRIPTION_LENGTH constants

* MM-11703: GOFMT

* MM-11703: Move upgrade code to version 5.3

* MM-11703: Comment back upgrade to 5.3.0

* MM-11703: Add API errors back

* MM-11703: Copy-Paste

model/incoming_webhook_test.go

@@ -80,12 +80,12 @@ func TestIncomingWebhookIsValid(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	o.Description = strings.Repeat("1", 129)
+	o.Description = strings.Repeat("1", 501)
 	if err := o.IsValid(); err == nil {
 		t.Fatal("should be invalid")
 	}
 
-	o.Description = strings.Repeat("1", 128)
+	o.Description = strings.Repeat("1", 500)
 	if err := o.IsValid(); err != nil {
 		t.Fatal(err)
 	}