Improve issue sidebar UI (#32587)

1. remove duplicate dividers
2. align reviewer items
3. merge & remove unused CSS styles


Before:

<details>


![image](https://github.com/user-attachments/assets/1b3121ee-b5fa-4fe9-b0f2-344d96dc5fbc)


![image](https://github.com/user-attachments/assets/ba8b97e6-114d-488c-adee-48f6c7a3b580)

</details>

After:

<details>


![image](https://github.com/user-attachments/assets/978eab3e-a5d7-4b68-90ce-079b61994d25)


![image](https://github.com/user-attachments/assets/a8b58a27-dd05-4c8d-be60-816439ce77c6)


![image](https://github.com/user-attachments/assets/b7e6a16c-bf98-4465-a805-9f4a642d366e)

</details>

templates/repo/issue/sidebar/reviewer_list.tmpl

@@ -17,7 +17,7 @@
 					<input type="text" placeholder="{{ctx.Locale.Tr "repo.issues.filter_reviewers"}}">
 				</div>
 			{{end}}
-			<div class="scrolling menu">
+			<div class="scrolling menu flex-items-menu">
 				{{range $data.Reviewers}}
 					{{if .User}}
 						<a class="item muted {{if .Requested}}checked{{end}}" href="{{.User.HomeLink}}" data-value="{{.ItemID}}" data-can-change="{{.CanChange}}"