fixes #4706 - adding back the ability to download a debug certificate

As part of an earlier commit to eliminate dead code, this
feature was accidentally removed.  Since is still needed
on the org edit page, it is being reintroduced.

A couple of notes:
- rather than bring back the old ui controller, the action
  is being added to the v2 api controller
- in order to reduce chance of the overrides being removed
  in the future, they are being placed in app/overrides/foreman