issue 2550536 adds the ability to readd an unlinked (removed) file or issue.

It would be good to add a verification step here as well. The remove buttons
for issues in the classic tracker often get removed to prevent unintended unlinking.

In https://sourceforge.net/p/roundup/mailman/message/37742787/ Thomas notes that the
ui has issues:

    There is a "remove" button next to each message, which
    - doesn't ask if the message should actually be removed, it just
      removes it from the issue without an easy way to undo mistakes.

In https://sourceforge.net/p/roundup/mailman/message/37743155/ I suggest:

   It would be nice to have a popup/redirect to _generic.unlink.html that
   says something like:

   ========================
    Remove file234 <insert filename here> from issue2345. Note this
    will not delete the data for the file. That data can be seen by
    going to <tracker_basepath>/file234.


    [remove file reference from issue] [cancel]

   ========================

   This would be great for the supplied tracker templates.

There are two ways to do this:

  1) use javascript to pop up a modal and get the user's ok there and post the unlink/remove.
     This keeps the same layout, just binds something else to the remove button. This is nice
     in that any changes the user may have made to the page are preserved since there is
     no link to a new page.
  2) don't use javascript. Link to @template=unlink and perform the form's post from there
     sending original issue and message/file designators. This could either transition
     to a new page (possibly loosing work), or open a popup in a new windows/tab like the
     current calendar, user search helpers do.

One other suggestion (From Ralf IIRC on that thread) was to not list the remove button
unless the user was the creator or the user has some additional privs (admin, is assigndto,
developer, caretaker etc). Adding an example on the wiki or in customizing.txt for this use 
case would be nice.