Merged design/admin2 into 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 ezflow extension.

I merged the contents of the `design/admin2` into `design/admin` - as the file `design/admin2/stylesheets/ezpage/ezpage.css` already has overriden the file `design/admin/stylesheets/ezpage/ezpage.css`, this file is overwritten and loses some unused CSS rules.

Cheers
:octocat: Jérôme