Add `Delete` button to label edit view

What does this MR do and why?

Related to #345195 (closed)

Adds a Delete button to the label edit view. This change affects Project, Group, and Admin label management.

Screenshots or screen recordings

Admin labels

Desktop

Before	After

Mobile

Before	After

Project labels

Desktop

Before	After

Mobile

Before	After

Group labels

Desktop

Before	After

Mobile

Before	After

How to set up and validate locally

Admin labels

1. Navigate to /admin/labels
2. Create a label
3. Edit the label

Project labels

1. Navigate to a project -> Project information -> Labels
2. Create a label
3. Edit the label

Group labels

1. Navigate to a group -> Group information -> Labels
2. Create a label
3. Edit the label

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

doc/user/admin_area/labels.md

@@ -7,10 +7,10 @@ type: reference
 
 # Labels administration **(FREE SELF)**
 
-Labels for the GitLab instance can be managed by selecting **Labels** (**{labels}**) from the Admin Area sidebar. For more details on how to manage labels, see [Labels](../project/labels.md).
+To manage labels for the GitLab instance, select **Labels** (**{labels}**) from the Admin Area sidebar. For more details on how to manage labels, see [Labels](../project/labels.md).
 
-Labels created in the Admin Area are automatically added to _new_ projects.
-Updating or adding labels in the Admin Area does not modify labels in _existing_ projects.
+Labels created in the Admin Area are automatically added to new projects.
+Updating or adding labels in the Admin Area does not modify labels in existing projects.
 
 ![Default label set](img/admin_labels_v14_7.png)