EZP-22271: Renamed design/admin2 to design/admin

Since in ezpublish-legacy, the the `design/admin2` has been renamed to `design/admin` (https://github.com/ezsystems/ezpublish-legacy/blob/master/doc/bc/5.0/changes-5.0.txt) this patch does the same for the ezautosave extension.

Corresponding JIRA issue: https://jira.ez.no/browse/EZP-22271

Cheers
:octocat: Jérôme