Fix link target validation in ExternalLinkBlock (#2102)

## Problem:

The validation in the ExternalLinkBlock was broken. The `isValid` check
uses `isLinkTarget` for validation:


https://github.com/vivid-planet/comet/blob/9867242fa6bad4849de58ccce1c28b816cc109a3/packages/admin/cms-admin/src/blocks/ExternalLinkBlock.tsx#L42-L44

The `targetUrl` field uses `validateUrl`:


https://github.com/vivid-planet/comet/blob/9867242fa6bad4849de58ccce1c28b816cc109a3/packages/admin/cms-admin/src/blocks/ExternalLinkBlock.tsx#L66-L73

The problem was that `validateUrl` internally used `isUrl` from class
validator. `isUrl` is less strict than `isLinkTarget`. This resulted in
no validation error in the UI but an error when trying to save the
block:


https://github.com/vivid-planet/comet/assets/13380047/a6c91ce1-94a5-4aac-a755-ef497a506504

## Solution:

I added `validateLinkTarget` using the stricter `isLinkTarget` check.

.changeset/six-meals-lie.md

@@ -0,0 +1,8 @@
+---
+"@comet/cms-admin": patch
+---
+
+Fix link target validation in `ExternalLinkBlock`
+
+Previously, two different validation checks were used.
+This resulted in an error when saving an invalid link target but no error message was shown.

packages/admin/cms-admin/src/blocks/ExternalLinkBlock.tsx

@@ -6,7 +6,7 @@ import { FormattedMessage } from "react-intl";
 
 import { ExternalLinkBlockData, ExternalLinkBlockInput } from "../blocks.generated";
 import { isLinkTarget } from "../validation/isLinkTarget";
-import { validateUrl } from "../validation/validateUrl";
+import { validateLinkTarget } from "../validation/validateLinkTarget";
 
 type State = ExternalLinkBlockData;
 
@@ -68,7 +68,7 @@ export const ExternalLinkBlock: BlockInterface<ExternalLinkBlockData, State, Ext
                         name="targetUrl"
                         component={FinalFormInput}
                         fullWidth
-                        validate={(url) => validateUrl(url)}
+                        validate={(url) => validateLinkTarget(url)}
                         disableContentTranslation
                     />
                     <Field name="openInNewWindow" type="checkbox">

packages/admin/cms-admin/src/validation/validateLinkTarget.tsx

@@ -0,0 +1,10 @@
+import React from "react";
+import { FormattedMessage } from "react-intl";
+
+import { isLinkTarget } from "./isLinkTarget";
+
+export function validateLinkTarget(linkTarget: string) {
+    if (!isLinkTarget(linkTarget)) {
+        return <FormattedMessage id="comet.validation.validateLinkTarget.invalid" defaultMessage="Invalid link target" />;
+    }
+}