[MM-25647]: ability to permanently delete users (#14944)

Summary:
    Config option to allow permanent user deletion ServiceSettings.EnableAPIUserDeletion
    Expose permanent user deletion through API
    Local mode for delete user for use in mmctl

Ticket Link:
    Server part of https://mattermost.atlassian.net/browse/MM-25647

api4/user.go

@@ -1193,7 +1193,16 @@ func deleteUser(c *Context, w http.ResponseWriter, r *http.Request) {
 	}
 	auditRec.AddMeta("user", user)
 
-	if _, err = c.App.UpdateActive(user, false); err != nil {
+	if c.Params.Permanent {
+		if *c.App.Config().ServiceSettings.EnableAPIUserDeletion {
+			err = c.App.PermanentDeleteUser(user)
+		} else {
+			err = model.NewAppError("deleteUser", "api.user.delete_user.not_enabled.app_error", nil, "userId="+c.Params.UserId, http.StatusUnauthorized)
+		}
+	} else {
+		_, err = c.App.UpdateActive(user, false)
+	}
+	if err != nil {
 		c.Err = err
 		return
 	}