Fixes #29445 - restrict STI deletion to admin (#7694)

When obsolete STI records deletion was introduced, we allowed even
non-admin users to delete that. This patch makes it possible only for
admins to delete records from the database. Non-admin users only see a
short message explaining what happened, that they should contact
administrator and request ID that can help administrator to figure out
what happened.

It was also necessary to add explicit require_login call, otherwise
User.current was nil in case the stack failed before user was loaded.
This also made the page looks much nicer, becase the menu can be
properly rendered.