fix(ui): simplify GitLab source setup view

Use the red incomplete-setup alert like GitHub, keep name + OAuth
credentials front-and-center, and tuck GitLab URL / API / SSH / system
wide options into an Advanced accordion for self-hosted users.

app/Livewire/Source/Gitlab/Change.php

@@ -59,6 +59,8 @@ class Change extends Component
 
     public ?string $oauthState = null;
 
+    private bool $shouldDeriveApiUrlAfterHtmlUrlUpdate = false;
+
     protected function rules(): array
     {
         return [
@@ -76,6 +78,19 @@ class Change extends Component
         ];
     }
 
+    public function updatingHtmlUrl(): void
+    {
+        $this->shouldDeriveApiUrlAfterHtmlUrlUpdate = blank($this->apiUrl)
+            || $this->apiUrl === rtrim($this->htmlUrl, '/').'/api/v4';
+    }
+
+    public function updatedHtmlUrl(): void
+    {
+        if ($this->shouldDeriveApiUrlAfterHtmlUrlUpdate) {
+            $this->apiUrl = rtrim($this->htmlUrl, '/').'/api/v4';
+        }
+    }
+
     public function mount()
     {
         try {