Only use supported sort order for "explore/users" page (#29430)

Thanks to inferenceus : some sort orders on the "explore/users" page
could list users by their lastlogintime/updatetime.

It leaks user's activity unintentionally. This PR makes that page only
use "supported" sort orders.

Removing the "sort orders" could also be a good solution, while IMO at
the moment keeping the "create time" and "name" orders is also fine, in
case some users would like to find a target user in the search result,
the "sort order" might help.

![image](ce5c39c1-1e86-484a-80c3-33cac6419af8)

(cherry picked from commit eedb8f41297c343d6073a7bab46e4df6ee297a90)

tests/integration/explore_user_test.go

@@ -0,0 +1,44 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package integration
+
+import (
+	"net/http"
+	"testing"
+
+	"code.gitea.io/gitea/tests"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestExploreUser(t *testing.T) {
+	defer tests.PrepareTestEnv(t)()
+
+	cases := []struct{ sortOrder, expected string }{
+		{"", "/explore/users?sort=newest&q="},
+		{"newest", "/explore/users?sort=newest&q="},
+		{"oldest", "/explore/users?sort=oldest&q="},
+		{"alphabetically", "/explore/users?sort=alphabetically&q="},
+		{"reversealphabetically", "/explore/users?sort=reversealphabetically&q="},
+	}
+	for _, c := range cases {
+		req := NewRequest(t, "GET", "/explore/users?sort="+c.sortOrder)
+		resp := MakeRequest(t, req, http.StatusOK)
+		h := NewHTMLParser(t, resp.Body)
+		href, _ := h.Find(`.ui.dropdown .menu a.active.item[href^="/explore/users"]`).Attr("href")
+		assert.Equal(t, c.expected, href)
+	}
+
+	// these sort orders shouldn't be supported, to avoid leaking user activity
+	cases404 := []string{
+		"/explore/users?sort=lastlogin",
+		"/explore/users?sort=reverselastlogin",
+		"/explore/users?sort=leastupdate",
+		"/explore/users?sort=reverseleastupdate",
+	}
+	for _, c := range cases404 {
+		req := NewRequest(t, "GET", c).SetHeader("Accept", "text/html")
+		MakeRequest(t, req, http.StatusNotFound)
+	}
+}