Bug 18591: Allow any number of comments on ILLs

Bug 18591: Allow any number of comments on ILLs

This patch makes it possible to add arbitrary comments to ILL
requests. Comments are read only and displayed in chronological
order. Comments can be added by librarians, but also added automatically
based on comments in the different protocols, so that comments from the
lending library can also be added.

To test:
- Enable the ILL module ("ILLModule" syspref + config in koha-conf.xml)
- Install the Dummy backend from here:
  https://github.com/PTFS-Europe/Dummy
- Create a Dummy ILL request
- Add comments to the request, checking that the name and borrowernumber
  of the person that added the comment is displayed ok
- Check that the displayed date and time the comment was submitted follows
  the "dateformat" syspref
- On the "List requests" page, check that the correct number of comments
  is displayed in the "Comments" column
- Check that the tests pass:
  prove t/db_dependent/Illcomments.t

Signed-off-by: andrew.isherwood@ptfs-europe.com
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>