Implement Wiki page redirections

This issue is based on the POC and comments in !56392 (closed). There are a couple of ways to implement redirections.

The first idea is to create a special file in the repository called _redirections, where users could add which source url and where the user should be redirected. This solution is faster to implement but it has several cons. More details about the possible implementation and pros/cons in !56392 (comment 528365936).

The other option would be to store those redirections in the database. We will have a new section in the wiki's menu where, once browsed, we could introduce the source url and the target url. The benefit from this is that it will be controlled by specific users and not like the former approach where anybody with write access to the repo could change redirections. We could also benefit from showing the redirections assigned to a specific page when we browse it.