Translations admins get no link to admin hidden distroseries translations

When a distroseries' translations are hidden, only Launchpad admins can see its translations overview page—and that is exactly the page with the link to the admin form that lets them un-hide the translations. They can still access the admin page by URL, but the link isn't shown.

The problem is in distroseries-langchart.pt, which requires launchpad.Admin privileges on the DistroSeries (not on DistroSeriesLanguage, which is what is checked elsewhere). Probably the cleanest solution is to move the check into the view and widen it to include Translations experts. The reminder for admins can then be shown regardless of who the user is.

This is a regression caused by the fix to bug 280647. It supersedes bug 116637, since fixing this one will fix that one as well.