[MM-37772] Idiomatic naming (URL, URI, API) (#18128)

* s/Url/URL/g & s/Uri/URI/g

* s/Api/API/g

model/outgoing_webhook.go

@@ -165,7 +165,7 @@ func (o *OutgoingWebhook) IsValid() *AppError {
 	}
 
 	for _, callback := range o.CallbackURLs {
-		if !IsValidHTTPUrl(callback) {
+		if !IsValidHTTPURL(callback) {
 			return NewAppError("OutgoingWebhook.IsValid", "model.outgoing_hook.is_valid.url.app_error", nil, "", http.StatusBadRequest)
 		}
 	}