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](https://github.com/go-gitea/gitea/assets/2114189/ce5c39c1-1e86-484a-80c3-33cac6419af8)

models/user/search.go

@@ -9,6 +9,7 @@ import (
 	"strings"
 
 	"code.gitea.io/gitea/models/db"
+	"code.gitea.io/gitea/modules/container"
 	"code.gitea.io/gitea/modules/structs"
 	"code.gitea.io/gitea/modules/util"
 
@@ -30,6 +31,8 @@ type SearchUserOptions struct {
 	Actor         *User // The user doing the search
 	SearchByEmail bool  // Search by email as well as username/full name
 
+	SupportedSortOrders container.Set[string] // if not nil, only allow to use the sort orders in this set
+
 	IsActive           util.OptionalBool
 	IsAdmin            util.OptionalBool
 	IsRestricted       util.OptionalBool