Admin page to MUI v6

[julisod]

Even thought I fixed the links to navigate to the correct page, there are some routes that are disabled (I assume it's because the pages under that don't work). Those pages are for editing the hackerpack, banner and organization. I didn't work on those because the admin page is not the priority right now, as discussed per our last meeting. But the default page works fine, with the addition of these buttons: